diff --git a/tools/cldr/cldr-to-icu/build-icu-data.xml b/tools/cldr/cldr-to-icu/build-icu-data.xml
index 2e1e3ceb32..82b00f331a 100644
--- a/tools/cldr/cldr-to-icu/build-icu-data.xml
+++ b/tools/cldr/cldr-to-icu/build-icu-data.xml
@@ -196,104 +196,116 @@
             <!-- The following elements configure directories in which a subset of the available
                  locales IDs should be generated. Unlike the main <localeId> element, these
                  filters must specify all locale IDs in full (but since they mostly select base
-                 languages, this isn't a big deal). -->
+                 languages, this isn't a big deal).
+
+                 As well as allowing some data directories to have a subset of available data (via
+                 the <localeIds> element) there are also mechanisms for controlling aliasing and
+                 the locale parent relation which allows the sharing of some ICU data in cases
+                 where it would otherwise need to be copied. The two mechanisms are:
+
+                 1: inheritLanguageSubtag: Used to rewrite the parent of a locale ID from "root" to
+                    its language subtag (e.g. "zh_Hant" has a natural parent of "root", but to allow
+                    some base language data to be shared it can be made to have a parent of "zh").
+
+                 2: forcedAlias: Used to add aliases for specific directories in order to affect the
+                    ICU behaviour in special cases.
+
+                 Between them these mechanisms are known as "tailorings" of the affected locales. -->
             <!-- TODO: Explain why these special cases are needed/different. -->
 
-            <directoryFilter dir="coll">
-                root,
+            <!-- Collation data is large, but also more sharable than other data, which is why there
+                 are a number of aliases and parent remappings for this directory. -->
+            <directory dir="coll" inheritLanguageSubtag="bs_Cyrl, sr_Latn, zh_Hant">
+                <!-- This alias is to avoid needing to copy and maintain the same collation data for
+                     "zh" and "yue". The maximized versions of "yue_Hans" is "yue_Hans_CN" (vs
+                     "zh_Hans_CN"), and for "yue" it's "yue_Hant_HK" (vs "zh_Hant_HK"), so the
+                     aliases are effectively just rewriting the base language. -->
+                <forcedAlias source="yue_Hans" target="zh_Hans"/>
+                <forcedAlias source="yue" target="zh_Hant"/>
+                <!-- TODO: Find out and document this properly. -->
+                <forcedAlias source="sr_ME" target="sr_Cyrl_ME"/>
 
-                // A-B
-                af, am, ars, ar, as, az, be, bg, bn, bo, bs_Cyrl, bs,
+                <localeIds>
+                    root,
 
-                // C-F
-                ca, ceb, chr, cs, cy, da, de_AT, de, dsb, dz, ee, el, en,
-                en_US_POSIX, en_US, eo, es, et, fa_AF, fa, fil, fi, fo, fr_CA, fr,
+                    // A-B
+                    af, am, ars, ar, as, az, be, bg, bn, bo, bs_Cyrl, bs,
 
-                // G-J
-                ga, gl, gu, ha, haw, he, hi, hr, hsb, hu, hy,
-                id_ID, id, ig, in, in_ID, is, it, iw_IL, iw, ja,
+                    // C-F
+                    ca, ceb, chr, cs, cy, da, de_AT, de, dsb, dz, ee, el, en,
+                    en_US_POSIX, en_US, eo, es, et, fa_AF, fa, fil, fi, fo, fr_CA, fr,
 
-                // K-P
-                ka, kk, kl, km, kn, kok, ko, ku, ky, lb, lkt, ln, lo, lt, lv,
-                mk, ml, mn, mo, mr, ms, mt, my, nb, ne, nl, nn, no_NO, no,
-                om, or, pa_IN, pa, pa_Guru, pl, ps, pt,
+                    // G-J
+                    ga, gl, gu, ha, haw, he, hi, hr, hsb, hu, hy,
+                    id_ID, id, ig, in, in_ID, is, it, iw_IL, iw, ja,
 
-                // R-T
-                ro, ru, se, sh_BA, sh_CS, sh, sh_YU, si, sk, sl, smn, sq,
-                sr_BA, sr_Cyrl_ME, sr_Latn, sr_ME, sr_RS, sr, sv, sw,
-                ta, te, th, tk, to, tr,
+                    // K-P
+                    ka, kk, kl, km, kn, kok, ko, ku, ky, lb, lkt, ln, lo, lt, lv,
+                    mk, ml, mn, mo, mr, ms, mt, my, nb, ne, nl, nn, no_NO, no,
+                    om, or, pa_IN, pa, pa_Guru, pl, ps, pt,
 
-                // U-Z
-                ug, uk, ur, uz, vi, wae, wo, xh, yi, yo, yue_CN, yue_Hans,
-                yue, zh_CN, zh_Hant, zh_HK, zh_MO, zh_SG, zh_TW, zh, zu
-            </directoryFilter>
+                    // R-T
+                    ro, ru, se, sh_BA, sh_CS, sh, sh_YU, si, sk, sl, smn, sq,
+                    sr_BA, sr_Cyrl_ME, sr_Latn, sr_ME, sr_RS, sr, sv, sw,
+                    ta, te, th, tk, to, tr,
 
-            <directoryFilter dir="rbnf">
-                root,
+                    // U-Z
+                    ug, uk, ur, uz, vi, wae, wo, xh, yi, yo, yue_CN, yue_Hans,
+                    yue, zh_CN, zh_Hant, zh_HK, zh_MO, zh_SG, zh_TW, zh, zu
+                </localeIds>
+            </directory>
 
-                // A-E
-                af, ak, am, ars, ar, az, be, bg, bs, ca, ccp, chr, cs, cy,
-                da, de_CH, de, ee, el, en_001, en_IN, en, eo, es_419, es_DO,
-                es_GT, es_HN, es_MX, es_NI, es_PA, es_PR, es_SV, es, es_US, et,
+            <directory dir="rbnf">
+                <!-- It is not at all clear why this is being done. It's certainly not exactly the
+                     same as above, since (a) the alias is reversed (b) "zh_Hant" does exist, with
+                     different data than "yue", so this alias is not just rewriting the base
+                     language. -->
+                <!-- TODO: Find out and document this properly. -->
+                <forcedAlias source="zh_Hant_HK" target="yue"/>
 
-                // F-P
-                fa_AF, fa, ff, fil, fi, fo, fr_BE, fr_CH, fr, ga, he, hi, hr,
-                hu, hy, id, in, is, it, iw, ja, ka, kl, km, ko, ky, lb,
-                lo, lrc, lt, lv, mk, ms, mt, my, nb, nl, nn, no, pl, pt_PT, pt,
+                <localeIds>
+                    root,
 
-                // Q-Z
-                qu, ro, ru, se, sh, sk, sl, sq, sr_Latn, sr, sv, sw, ta, th, tr,
-                uk, vi, yue_Hans, yue, zh_Hant_HK, zh_Hant, zh_HK, zh_MO, zh_TW, zh
-            </directoryFilter>
+                    // A-E
+                    af, ak, am, ars, ar, az, be, bg, bs, ca, ccp, chr, cs, cy,
+                    da, de_CH, de, ee, el, en_001, en_IN, en, eo, es_419, es_DO,
+                    es_GT, es_HN, es_MX, es_NI, es_PA, es_PR, es_SV, es, es_US, et,
 
-            <directoryFilter dir="brkitr">
-                root,
-                de, el, en, en_US_POSIX, en_US, es, fr, it, ja, pt, ru, zh_Hant, zh
-            </directoryFilter>
+                    // F-P
+                    fa_AF, fa, ff, fil, fi, fo, fr_BE, fr_CH, fr, ga, he, hi, hr,
+                    hu, hy, id, in, is, it, iw, ja, ka, kl, km, ko, ky, lb,
+                    lo, lrc, lt, lv, mk, ms, mt, my, nb, nl, nn, no, pl, pt_PT, pt,
 
-            <!-- The following elements configure some very special case locale alias behaviour,
-                 mainly to support situations where the natural alias relationship is not wanted
-                 for a particular type of data. -->
+                    // Q-Z
+                    qu, ro, ru, se, sh, sk, sl, sq, sr_Latn, sr, sv, sw, ta, th, tr,
+                    uk, vi, yue_Hans, yue, zh_Hant_HK, zh_Hant, zh_HK, zh_MO, zh_TW, zh
+                </localeIds>
+            </directory>
+
+            <directory dir="brkitr" inheritLanguageSubtag="zh_Hant">
+                <localeIds>
+                    root,
+                    de, el, en, en_US_POSIX, en_US, es, fr, it, ja, pt, ru, zh_Hant, zh
+                </localeIds>
+            </directory>
 
             <!-- GLOBAL ALIASES -->
 
             <!-- Some spoken languages (e.g. "ars") inherit all their data from a written language
                  (e.g. "ar_SA"). However CLDR doesn't currently support a way to represent that
                  relationship. Unlike deprecated languages for which an alias can be inferred from
-                 the "languageAlias" element, there's no way in CLDR to represent the fact that we
-                 want "ars" (a non-deprecated language) to inherit the data of "ar_SA".
+                 the "languageAlias" CLDR data, there's no way in CLDR to represent the fact that
+                 we want "ars" (a non-deprecated language) to inherit the data of "ar_SA".
 
                  This alias is the first example of potentially many cases where ICU needs to
-                 generate an alias in order to affect "sideways inheritence" for spoken languages,
-                 and at some stage it should be supported properly in the CLDR data. -->
+                 generate an alias in order to affect "sideways inheritance" for spoken languages,
+                 and at some stage it should probably be supported properly in the CLDR data. -->
             <forcedAlias source="ars" target="ar_SA"/>
 
             <!-- A legacy global alias (note that "no_NO_NY" is not even structurally valid). -->
             <forcedAlias source="no_NO_NY" target="nn_NO"/>
 
-            <!-- PER-DIRECTORY ALIASES (these are really special cases) -->
-
-            <!-- It is not at all clear why this is being done (we expect "sr_Latn_ME" normally). -->
-            <!-- TODO: Find out and document this properly. -->
-            <forcedAlias dir="coll" source="sr_ME" target="sr_Cyrl_ME"/>
-
-            <!-- This alias is to avoid needing to copy and maintain the same "zh" data for "yue".
-                 The maximized versions of "yue_Hans" is "yue_Hans_CN" (vs "zh_Hans_CN"), and for
-                 "yue" it's "yue_Hant_HK" (vs "zh_Hant_HK"), so the aliases are effectively just
-                 rewriting the base language.
-
-                 This is similar to the case for "ars"/"ar_SA" but it is not done globally, since
-                 CLDR data does exist for "yue" and "yue_Hans" which is NOT the same as "zh_Hant"
-                 and "zh_Hans"/"zh". This mapping is a bit more of a "hack" for the purposes of
-                 reducing data duplication in ICU. -->
-            <forcedAlias dir="coll" source="yue_Hans" target="zh_Hans"/>
-            <forcedAlias dir="coll" source="yue" target="zh_Hant"/>
-
-            <!-- It is not at all clear why this is being done. It's certainly not exactly the same
-                 as above, since (a) the alias is reversed (b) "zh_Hant" does exist, with different
-                 data than "yue", so this alias is not just rewriting the base language. -->
-            <!-- TODO: Find out and document this properly. -->
-            <forcedAlias dir="rbnf" source="zh_Hant_HK" target="yue"/>
+            <!-- ALTERNATE VALUES -->
 
             <!-- The following elements configure alternate values for some special case paths.
                  The target path will only be replaced if both it, and the source path, exist in
diff --git a/tools/cldr/cldr-to-icu/src/main/java/org/unicode/icu/tool/cldrtoicu/IcuConverterConfig.java b/tools/cldr/cldr-to-icu/src/main/java/org/unicode/icu/tool/cldrtoicu/IcuConverterConfig.java
index 519b4f731e..ac12c04819 100644
--- a/tools/cldr/cldr-to-icu/src/main/java/org/unicode/icu/tool/cldrtoicu/IcuConverterConfig.java
+++ b/tools/cldr/cldr-to-icu/src/main/java/org/unicode/icu/tool/cldrtoicu/IcuConverterConfig.java
@@ -45,6 +45,7 @@ public final class IcuConverterConfig implements LdmlConverterConfig {
         private boolean emitReport = false;
         private final SetMultimap<IcuLocaleDir, String> localeIdsMap = TreeMultimap.create();
         private final Table<IcuLocaleDir, String, String> forcedAliases = TreeBasedTable.create();
+        private final Table<IcuLocaleDir, String, String> forcedParents = TreeBasedTable.create();
 
         /**
          * Sets the output directory in which the ICU data directories and files will go. This is
@@ -95,6 +96,11 @@ public final class IcuConverterConfig implements LdmlConverterConfig {
             return this;
         }
 
+        public Builder addForcedParent(IcuLocaleDir dir, String localeId, String parent) {
+            forcedParents.put(dir, localeId, parent);
+            return this;
+        }
+
         /** Returns a converter config from the current builder state. */
         public LdmlConverterConfig build() {
             return new IcuConverterConfig(this);
@@ -109,6 +115,7 @@ public final class IcuConverterConfig implements LdmlConverterConfig {
     private final ImmutableSet<String> allLocaleIds;
     private final ImmutableSetMultimap<IcuLocaleDir, String> localeIdsMap;
     private final ImmutableTable<IcuLocaleDir, String, String> forcedAliases;
+    private final ImmutableTable<IcuLocaleDir, String, String> forcedParents;
 
     private IcuConverterConfig(Builder builder) {
         this.outputDir = checkNotNull(builder.outputDir);
@@ -128,6 +135,7 @@ public final class IcuConverterConfig implements LdmlConverterConfig {
         this.allLocaleIds = ImmutableSet.copyOf(builder.localeIdsMap.values());
         this.localeIdsMap = ImmutableSetMultimap.copyOf(builder.localeIdsMap);
         this.forcedAliases = ImmutableTable.copyOf(builder.forcedAliases);
+        this.forcedParents = ImmutableTable.copyOf(builder.forcedParents);
     }
 
     public static Builder builder() {
@@ -164,6 +172,11 @@ public final class IcuConverterConfig implements LdmlConverterConfig {
         return forcedAliases.row(dir);
     }
 
+    @Override
+    public ImmutableMap<String, String> getForcedParents(IcuLocaleDir dir) {
+        return forcedParents.row(dir);
+    }
+
     @Override public ImmutableSet<String> getAllLocaleIds() {
         return allLocaleIds;
     }
diff --git a/tools/cldr/cldr-to-icu/src/main/java/org/unicode/icu/tool/cldrtoicu/LdmlConverter.java b/tools/cldr/cldr-to-icu/src/main/java/org/unicode/icu/tool/cldrtoicu/LdmlConverter.java
index d009cec3d7..6d2d600465 100644
--- a/tools/cldr/cldr-to-icu/src/main/java/org/unicode/icu/tool/cldrtoicu/LdmlConverter.java
+++ b/tools/cldr/cldr-to-icu/src/main/java/org/unicode/icu/tool/cldrtoicu/LdmlConverter.java
@@ -323,11 +323,16 @@ public final class LdmlConverter {
                 // The split data can still be empty for this directory, but that's expected (it
                 // might only be written because it has an explicit parent added below).
                 splitPaths.get(dir).forEach(p -> splitData.add(p, icuData.get(p)));
-                // If we add an explicit parent locale, it forces the data to be written.
-                parent.ifPresent(p -> {
-                    splitData.add(RB_PARENT, p);
-                    graphMetadata.get(dir).addParent(id, p);
-                });
+
+                // If we add an explicit parent locale, it forces the data to be written. This is
+                // where we check for forced overrides of the parent relationship (which is a per
+                // directory thing).
+                parent
+                    .map(p -> config.getForcedParents(dir).getOrDefault(id, p))
+                    .ifPresent(p -> {
+                        splitData.add(RB_PARENT, p);
+                        graphMetadata.get(dir).addParent(id, p);
+                    });
 
                 if (!splitData.getPaths().isEmpty() || isBaseLanguage || dir.includeEmpty()) {
                     splitData.setVersion(CldrDataSupplier.getCldrVersionString());
@@ -384,7 +389,7 @@ public final class LdmlConverter {
 
         Map<String, String> aliasMap = new LinkedHashMap<>();
         for (String id : localeIds) {
-            if (forcedAliases.keySet().contains(id)) {
+            if (forcedAliases.containsKey(id)) {
                 // Forced aliases will be added later and don't need to be processed here. This
                 // is especially necessary if the ID is not structurally valid (e.g. "no_NO_NY")
                 // since that cannot be processed by the code below.
diff --git a/tools/cldr/cldr-to-icu/src/main/java/org/unicode/icu/tool/cldrtoicu/LdmlConverterConfig.java b/tools/cldr/cldr-to-icu/src/main/java/org/unicode/icu/tool/cldrtoicu/LdmlConverterConfig.java
index c42b11ae70..de7252c518 100644
--- a/tools/cldr/cldr-to-icu/src/main/java/org/unicode/icu/tool/cldrtoicu/LdmlConverterConfig.java
+++ b/tools/cldr/cldr-to-icu/src/main/java/org/unicode/icu/tool/cldrtoicu/LdmlConverterConfig.java
@@ -96,13 +96,22 @@ public interface LdmlConverterConfig {
     Set<String> getTargetLocaleIds(IcuLocaleDir dir);
 
     /**
-     * Return a map of locale IDs which specifies aliases which are applied to the given
-     * directory in contradiction to the natural alias or parent ID which would otherwise
-     * be generated. This is a mechanism for restructuring the parent chain and linking
-     * locales together in non-standard and unexpected ways.
+     * Returns a map of locale IDs which specifies aliases which are applied to the given directory
+     * in contradiction to the natural alias which would otherwise be generated. This mechanism
+     * allows for restructuring locale relationships on a per directory basis for special-case
+     * behaviour (such as sharing data which would otherwise need to be copied).
      */
     Map<String, String> getForcedAliases(IcuLocaleDir dir);
 
+    /**
+     * Returns a map of locale IDs which specifies aliases which are applied to the given directory
+     * in contradiction to the natural parent which would otherwise be generated. This mechanism
+     * allows for restructuring locale relationships on a per directory basis for special-case
+     * behaviour (such as sharing data which would otherwise need to be copied).
+     */
+    // TODO: Combine this and the force aliases into a single mechanism at this level.
+    Map<String, String> getForcedParents(IcuLocaleDir dir);
+
     /**
      * Whether to emit a summary report for debug purposes after conversion is complete.
      */
diff --git a/tools/cldr/cldr-to-icu/src/main/java/org/unicode/icu/tool/cldrtoicu/ant/ConvertIcuDataTask.java b/tools/cldr/cldr-to-icu/src/main/java/org/unicode/icu/tool/cldrtoicu/ant/ConvertIcuDataTask.java
index fbd3c52fac..503016aaee 100644
--- a/tools/cldr/cldr-to-icu/src/main/java/org/unicode/icu/tool/cldrtoicu/ant/ConvertIcuDataTask.java
+++ b/tools/cldr/cldr-to-icu/src/main/java/org/unicode/icu/tool/cldrtoicu/ant/ConvertIcuDataTask.java
@@ -65,9 +65,10 @@ public final class ConvertIcuDataTask extends Task {
     private Path cldrPath;
     private CldrDraftStatus minimumDraftStatus;
     // Set of default locale ID specifiers (wildcard IDs which are expanded).
-    private ImmutableSet<String> localeIdSpec;
+    private LocaleIds localeIds = null;
     // Per directory overrides (fully specified locale IDs).
     private final SetMultimap<IcuLocaleDir, String> perDirectoryIds = HashMultimap.create();
+    private final SetMultimap<IcuLocaleDir, String> inheritLanguageSubtag = HashMultimap.create();
     private final IcuConverterConfig.Builder config = IcuConverterConfig.builder();
     // Don't try and resolve actual paths until inside the execute method.
     private final List<AltPath> altPaths = new ArrayList<>();
@@ -136,9 +137,11 @@ public final class ConvertIcuDataTask extends Task {
         }
     }
 
-    public static final class DirectoryFilter extends Task {
+    public static final class Directory extends Task {
         private IcuLocaleDir dir;
-        private ImmutableSet<String> ids;
+        private ImmutableSet<String> inheritLanguageSubtag = ImmutableSet.of();
+        private final List<ForcedAlias> forcedAliases = new ArrayList<>();
+        private LocaleIds localeIds = null;
 
         @SuppressWarnings("unused")
         public void setDir(String directory) {
@@ -146,27 +149,33 @@ public final class ConvertIcuDataTask extends Task {
         }
 
         @SuppressWarnings("unused")
-        public void addText(String localeIds) {
-            this.ids = parseLocaleIds(localeIds);
+        public void setInheritLanguageSubtag(String localeIds) {
+            this.inheritLanguageSubtag = parseLocaleIds(localeIds);
+        }
+
+        @SuppressWarnings("unused")
+        public void addConfiguredForcedAlias(ForcedAlias alias) {
+            forcedAliases.add(alias);
+        }
+
+        @SuppressWarnings("unused")
+        public void addConfiguredLocaleIds(LocaleIds localeIds) {
+            checkBuild(this.localeIds == null,
+                "Cannot add more that one <localeIds> element for <directory>: %s", dir);
+            this.localeIds =  localeIds;
         }
 
         @Override
         public void init() throws BuildException {
-            checkBuild(dir != null, "Directory must be specified");
-            checkBuild(!ids.isEmpty(), "Locale IDs must be specified");
+            checkBuild(dir != null, "Directory attribute 'dir' must be specified");
+            checkBuild(localeIds != null, "<localeIds> must be specified for <directory>: %s", dir);
         }
     }
 
     public static final class ForcedAlias extends Task {
-        private Optional<IcuLocaleDir> dir = Optional.empty();
         private String source = "";
         private String target = "";
 
-        @SuppressWarnings("unused")
-        public void setDir(String directory) {
-            this.dir = resolveDir(directory);
-        }
-
         @SuppressWarnings("unused")
         public void setSource(String source) {
             this.source = whitespace().trimFrom(source);
@@ -184,7 +193,6 @@ public final class ConvertIcuDataTask extends Task {
         }
     }
 
-
     public static final class AltPath extends Task {
         private String source = "";
         private String target = "";
@@ -214,23 +222,22 @@ public final class ConvertIcuDataTask extends Task {
 
     @SuppressWarnings("unused")
     public void addConfiguredLocaleIds(LocaleIds localeIds) {
-        checkBuild(this.localeIdSpec == null, "Cannot add more that one <localeIds> element");
-        this.localeIdSpec =  localeIds.ids;
+        checkBuild(this.localeIds == null, "Cannot add more that one <localeIds> element");
+        this.localeIds =  localeIds;
     }
 
     @SuppressWarnings("unused")
-    public void addConfiguredDirectoryFilter(DirectoryFilter filter) {
-        perDirectoryIds.putAll(filter.dir, filter.ids);
+    public void addConfiguredDirectory(Directory filter) {
+        perDirectoryIds.putAll(filter.dir, filter.localeIds.ids);
+        inheritLanguageSubtag.putAll(filter.dir, filter.inheritLanguageSubtag);
+        filter.forcedAliases.forEach(a -> config.addForcedAlias(filter.dir, a.source, a.target));
     }
 
+    // Aliases on the outside are applied to all directories.
     @SuppressWarnings("unused")
     public void addConfiguredForcedAlias(ForcedAlias alias) {
-        if (alias.dir.isPresent()) {
-            config.addForcedAlias(alias.dir.get(), alias.source, alias.target);
-        } else {
-            for (IcuLocaleDir dir : IcuLocaleDir.values()) {
-                config.addForcedAlias(dir, alias.source, alias.target);
-            }
+        for (IcuLocaleDir dir : IcuLocaleDir.values()) {
+            config.addForcedAlias(dir, alias.source, alias.target);
         }
     }
 
@@ -244,6 +251,8 @@ public final class ConvertIcuDataTask extends Task {
 
     @SuppressWarnings("unused")
     public void execute() throws BuildException {
+        checkBuild(localeIds != null, "<localeIds> must be specified");
+
         CldrDataSupplier src = CldrDataSupplier
             .forCldrFilesIn(cldrPath)
             .withDraftStatusAtLeast(minimumDraftStatus);
@@ -260,10 +269,20 @@ public final class ConvertIcuDataTask extends Task {
 
         SupplementalData supplementalData = SupplementalData.create(src);
         ImmutableSet<String> defaultTargetIds =
-            LocaleIdResolver.expandTargetIds(this.localeIdSpec, supplementalData);
+            LocaleIdResolver.expandTargetIds(this.localeIds.ids, supplementalData);
         for (IcuLocaleDir dir : IcuLocaleDir.values()) {
             Iterable<String> ids = perDirectoryIds.asMap().getOrDefault(dir, defaultTargetIds);
             config.addLocaleIds(dir, Iterables.filter(ids, idFilter::test));
+
+            // We should only have locale IDs like "zh_Hant" here (language + script) and only
+            // those which would naturally inherit to "root"
+            inheritLanguageSubtag.get(dir).forEach(id -> {
+                checkArgument(id.matches("[a-z]{2}_[A-Z][a-z]{3}"),
+                    "Invalid locale ID for inheritLanguageSubtag (expect '<lang>_<Script>'): ", id);
+                checkArgument(supplementalData.getParent(id).equals("root"),
+                    "Invalid locale ID for inheritLanguageSubtag (parent must be 'root'): ", id);
+                config.addForcedParent(dir, id, id.substring(0, 2));
+            });
         }
         config.setMinimumDraftStatus(minimumDraftStatus);
         LdmlConverter.convert(src, supplementalData, config.build());