diff --git a/tools/dotcms-cli/api-data-model/src/main/java/com/dotcms/api/LanguageAPI.java b/tools/dotcms-cli/api-data-model/src/main/java/com/dotcms/api/LanguageAPI.java
index 1a571db5b133..14c94da4ebd5 100644
--- a/tools/dotcms-cli/api-data-model/src/main/java/com/dotcms/api/LanguageAPI.java
+++ b/tools/dotcms-cli/api-data-model/src/main/java/com/dotcms/api/LanguageAPI.java
@@ -8,6 +8,7 @@
 import com.dotcms.model.views.CommonViews.LanguageFileView;
 import com.fasterxml.jackson.annotation.JsonView;
 import java.util.List;
+import java.util.Map;
 import jakarta.ws.rs.Consumes;
 import jakarta.ws.rs.DELETE;
 import jakarta.ws.rs.GET;
@@ -120,4 +121,14 @@ ResponseEntityView<Language> update(@PathParam("languageId") String languageId,
             summary = " Deletes an existing language from the system"
     )
     ResponseEntityView<String> delete(@PathParam("languageId") String languageId);
+
+    @PUT
+    @Path("/{languageId}/_makedefault")
+    @Operation(
+            summary = " Makes the language with the given id the default language. The body field "
+                    + "'fireTransferAssetsJob' controls whether a background job is scheduled to "
+                    + "transfer assets from the previous default language."
+    )
+    ResponseEntityView<Language> makeDefault(@PathParam("languageId") String languageId,
+            Map<String, Boolean> form);
 }
diff --git a/tools/dotcms-cli/api-data-model/src/test/java/com/dotcms/api/SiteAPIIT.java b/tools/dotcms-cli/api-data-model/src/test/java/com/dotcms/api/SiteAPIIT.java
index 2be7b2246fbd..bf74da3366b2 100644
--- a/tools/dotcms-cli/api-data-model/src/test/java/com/dotcms/api/SiteAPIIT.java
+++ b/tools/dotcms-cli/api-data-model/src/test/java/com/dotcms/api/SiteAPIIT.java
@@ -17,6 +17,7 @@
 import java.io.IOException;
 import java.net.URL;
 import java.util.List;
+import java.util.Map;
 import java.util.function.Supplier;
 import jakarta.inject.Inject;
 import jakarta.ws.rs.NotFoundException;
@@ -63,12 +64,21 @@ public void setupTest() throws IOException {
     }
 
     /**
-     * Warms the dotCMS language cache by listing languages, retrying up to 3 times to
-     * cover the race window right after a fresh dotCMS start where the default-language
-     * lookup can briefly return a non-positive id. When that happens, downstream site
-     * creation fails with HTTP 500 "Language cannot be null" because the contentlet ends
-     * up with languageId <= 0 and a unique-field validation tries to resolve it back to
-     * a Language. See issue #35780.
+     * Ensures the dotCMS test environment has a real default language (id &gt; 0) before
+     * the SiteAPIIT tests run. Without this, {@code POST /api/v1/site} resolves the Host
+     * contentlet's languageId from {@code getDefaultLanguage()} — which in some CI test
+     * environments returns the {@code LANG__404} sentinel with id=-1, breaking the
+     * downstream unique-field validation with HTTP 500 "Language cannot be null".
+     * See issue #35780.
+     *
+     * <p>Strategy:
+     * <ol>
+     *   <li>List languages</li>
+     *   <li>If the entry marked {@code defaultLanguage=true} has a valid id, we are done.</li>
+     *   <li>Otherwise pick the first language with id &gt; 0 (English in starter data)
+     *       and call {@code PUT /api/v2/languages/{id}/_makedefault} to fix the broken
+     *       default. {@code fireTransferAssetsJob=false} so this is a fast metadata change.</li>
+     * </ol>
      */
     private void warmLanguageCacheIfNeeded() {
         if (languageCacheWarmed) {
@@ -77,15 +87,19 @@ private void warmLanguageCacheIfNeeded() {
         final int maxAttempts = 3;
         for (int attempt = 1; attempt <= maxAttempts; attempt++) {
             try {
-                final ResponseEntityView<List<Language>> response =
-                        clientFactory.getClient(LanguageAPI.class).list();
-                final List<Language> languages = response != null ? response.entity() : null;
-                final boolean hasValidLanguage = languages != null && languages.stream()
-                        .anyMatch(l -> l != null && l.id().isPresent() && l.id().get() > 0);
-                if (hasValidLanguage) {
+                final List<Language> languages = listLanguages();
+                if (hasValidDefault(languages)) {
                     languageCacheWarmed = true;
                     return;
                 }
+                final Long fallbackId = pickFallbackLanguageId(languages);
+                if (fallbackId != null && tryMakeDefault(fallbackId)) {
+                    // Re-list to confirm the fix took before declaring success.
+                    if (hasValidDefault(listLanguages())) {
+                        languageCacheWarmed = true;
+                        return;
+                    }
+                }
             } catch (Exception ignored) {
                 // fall through to retry
             }
@@ -98,6 +112,44 @@ private void warmLanguageCacheIfNeeded() {
         }
     }
 
+    private List<Language> listLanguages() {
+        final ResponseEntityView<List<Language>> response =
+                clientFactory.getClient(LanguageAPI.class).list();
+        return response != null ? response.entity() : null;
+    }
+
+    private static boolean hasValidDefault(final List<Language> languages) {
+        if (languages == null) {
+            return false;
+        }
+        return languages.stream().anyMatch(l ->
+                l != null
+                        && l.id().isPresent() && l.id().get() > 0
+                        && l.defaultLanguage().orElse(false));
+    }
+
+    private static Long pickFallbackLanguageId(final List<Language> languages) {
+        if (languages == null) {
+            return null;
+        }
+        return languages.stream()
+                .filter(l -> l != null && l.id().isPresent() && l.id().get() > 0)
+                .map(l -> l.id().get())
+                .findFirst()
+                .orElse(null);
+    }
+
+    private boolean tryMakeDefault(final Long languageId) {
+        try {
+            clientFactory.getClient(LanguageAPI.class)
+                    .makeDefault(String.valueOf(languageId),
+                            Map.of("fireTransferAssetsJob", Boolean.FALSE));
+            return true;
+        } catch (Exception ignored) {
+            return false;
+        }
+    }
+
     /**
      * Retries the supplied Site create call when the server returns HTTP 500
      * "Language cannot be null". The {@link #warmLanguageCacheIfNeeded()} warmup