diff --git a/.changeset/frank-days-start.md b/.changeset/frank-days-start.md
new file mode 100644
index 0000000000..02ec1ebd41
--- /dev/null
+++ b/.changeset/frank-days-start.md
@@ -0,0 +1,5 @@
+---
+"@medusajs/translation": patch
+---
+
+fix(translation): prevent duplicate locale error on server restart
diff --git a/packages/modules/translation/src/loaders/defaults.ts b/packages/modules/translation/src/loaders/defaults.ts
index ab05f516f6..1509127908 100644
--- a/packages/modules/translation/src/loaders/defaults.ts
+++ b/packages/modules/translation/src/loaders/defaults.ts
@@ -3,7 +3,10 @@ import {
   Logger,
   ModulesSdkTypes,
 } from "@medusajs/framework/types"
-import { ContainerRegistrationKeys } from "@medusajs/framework/utils"
+import {
+  ContainerRegistrationKeys,
+  normalizeLocale,
+} from "@medusajs/framework/utils"
 import Locale from "@models/locale"
 
 /**
@@ -70,7 +73,24 @@ export default async ({ container }: LoaderOptions): Promise<void> => {
     container.resolve("localeService")
 
   try {
-    const resp = await localeService_.upsert(defaultLocales)
+    // Fetch existing locales to map their IDs for upsert
+    // The upsert method uses `id` as the key, so we need to include IDs for existing locales
+    const existingLocales = await localeService_.list(
+      {},
+      { select: ["id", "code"] }
+    )
+    const existingByCode = new Map(
+      existingLocales.map((l) => [l.code, l.id])
+    )
+
+    // Map default locales to include IDs for existing ones
+    const localesToUpsert = defaultLocales.map((locale) => {
+      const normalizedCode = normalizeLocale(locale.code)
+      const existingId = existingByCode.get(normalizedCode)
+      return existingId ? { ...locale, id: existingId } : locale
+    })
+
+    const resp = await localeService_.upsert(localesToUpsert)
     logger.debug(`Loaded ${resp.length} locales`)
   } catch (error) {
     logger.warn(