diff --git a/toolkit/components/nimbus/lib/ExperimentManager.sys.mjs b/toolkit/components/nimbus/lib/ExperimentManager.sys.mjs
index 46c946081a4b..be1372dc7086 100644
--- a/toolkit/components/nimbus/lib/ExperimentManager.sys.mjs
+++ b/toolkit/components/nimbus/lib/ExperimentManager.sys.mjs
@@ -414,9 +414,12 @@ export class _ExperimentManager {
       lastSeen: new Date().toJSON(),
       featureIds,
       prefs,
-      localizations,
     };
 
+    if (localizations) {
+      experiment.localizations = localizations;
+    }
+
     if (typeof isRollout !== "undefined") {
       experiment.isRollout = isRollout;
     }
diff --git a/toolkit/components/nimbus/schemas/NimbusEnrollment.schema.json b/toolkit/components/nimbus/schemas/NimbusEnrollment.schema.json
index 13d568b89c6e..9f13c9094319 100644
--- a/toolkit/components/nimbus/schemas/NimbusEnrollment.schema.json
+++ b/toolkit/components/nimbus/schemas/NimbusEnrollment.schema.json
@@ -209,6 +209,16 @@
             },
             "additionalProperties": false
           }
+        },
+        "localizations": {
+          "type": "object",
+          "additionalProperties": {
+            "type": "object",
+            "additionalProperties": {
+              "type": "string"
+            }
+          },
+          "description": "Per-locale localization substitutions.\n\nThe top level key is the locale (e.g., \"en-US\" or \"fr\"). Each entry is a mapping of string IDs to their localized equivalents.\n\nOnly supported on desktop."
         }
       },
       "required": [