diff --git a/.changeset/metal-bottles-travel.md b/.changeset/metal-bottles-travel.md
new file mode 100644
index 000000000..9bd7dd5f8
--- /dev/null
+++ b/.changeset/metal-bottles-travel.md
@@ -0,0 +1,5 @@
+---
+"@lingo.dev/_spec": patch
+---
+
+Add unit test for utility function in locales.ts
diff --git a/packages/spec/src/locales.spec.ts b/packages/spec/src/locales.spec.ts
index 0291979c7..1f679b4d2 100644
--- a/packages/spec/src/locales.spec.ts
+++ b/packages/spec/src/locales.spec.ts
@@ -1,5 +1,5 @@
 import { describe, it, expect } from "vitest";
-import { normalizeLocale } from "./locales";
+import { getLocaleCodeDelimiter, normalizeLocale, resolveLocaleCode, resolveOverriddenLocale } from "./locales";
 
 describe("normalizeLocale", () => {
   it("should return normalized locale for short locale codes", () => {
@@ -24,3 +24,61 @@ describe("normalizeLocale", () => {
     expect(normalizeLocale("zh-rCN")).toEqual("zh-CN");
   });
 });
+
+describe("resolveLocaleCode", () => {
+  it("should resolve a short locale code to the first full locale code in the map", () => {
+    expect(resolveLocaleCode("en")).toEqual("en-US");
+    expect(resolveLocaleCode("fr")).toEqual("fr-FR");
+    expect(resolveLocaleCode("az")).toEqual("az-AZ");
+  });
+
+  it("should return the full locale code if it is already provided", () => {
+    expect(resolveLocaleCode("en-US")).toEqual("en-US");
+    expect(resolveLocaleCode("fr-CA")).toEqual("fr-CA");
+    expect(resolveLocaleCode("es-MX")).toEqual("es-MX");
+  });
+
+  it("should throw an error for an invalid or unsupported locale code", () => {
+    expect(() => resolveLocaleCode("az-US")).toThrow("Invalid locale code");
+    expect(() => resolveLocaleCode("au")).toThrow("Invalid locale code");
+  });
+
+  it("should return first code for locales with multiple variants", () => {
+    expect(resolveLocaleCode("sr")).toEqual("sr-RS");
+    expect(resolveLocaleCode("zh")).toEqual("zh-CN");
+  });
+});
+
+describe("getLocaleCodeDelimiter", () => {
+  it("should return '-' for locale codes with hyphen delimiter", () => {
+    expect(getLocaleCodeDelimiter("en-US")).toEqual("-");
+    expect(getLocaleCodeDelimiter("fr-FR")).toEqual("-");
+  });
+
+  it("should return '_' for locale codes with underscore delimiter", () => {
+    expect(getLocaleCodeDelimiter("en_US")).toEqual("_");
+    expect(getLocaleCodeDelimiter("fr_FR")).toEqual("_");
+  });
+
+  it("should return undefined for locale codes without a recognized delimiter", () => {
+    expect(getLocaleCodeDelimiter("enUS")).toBeNull();
+    expect(getLocaleCodeDelimiter("frFR")).toBeNull();
+  });
+});
+
+describe("resolveOverridenLocale", () => {
+  it("should return the same locale if no delimiter is provided", () => {
+    expect(resolveOverriddenLocale("en-US")).toEqual("en-US");
+    expect(resolveOverriddenLocale("fr_FR")).toEqual("fr_FR");
+  });
+
+  it("should replace the delimiter with the specified one", () => {
+    expect(resolveOverriddenLocale("en-US", "_")).toEqual("en_US");
+    expect(resolveOverriddenLocale("fr_FR", "-")).toEqual("fr-FR");
+  });
+
+  it("should return the same locale if no recognized delimiter is found", () => {
+    expect(resolveOverriddenLocale("enUS", "_")).toEqual("enUS");
+    expect(resolveOverriddenLocale("frFR", "-")).toEqual("frFR");
+  });
+});
diff --git a/packages/spec/src/locales.ts b/packages/spec/src/locales.ts
index 29703633a..d19e14568 100644
--- a/packages/spec/src/locales.ts
+++ b/packages/spec/src/locales.ts
@@ -222,7 +222,6 @@ export const localeCodeSchema = Z.string().refine((value) => localeCodes.include
   message: "Invalid locale code",
 });
 
-
 /**
  * Resolves a locale code to its full locale representation.
  *
@@ -234,9 +233,7 @@ export const localeCodeSchema = Z.string().refine((value) => localeCodes.include
  * @return {LocaleCodeFull} The resolved full locale code
  * @throws {Error} If the provided locale code is invalid.
  */
-
-
-export const resolveLocaleCode = (value: LocaleCode): LocaleCodeFull => {
+export const resolveLocaleCode = (value: string): LocaleCodeFull => {
   const existingFullLocaleCode = Object.values(localeMap)
     .flat()
     .includes(value as any);