diff --git a/tests/setLanguage.test.ts b/tests/setLanguage.test.ts
new file mode 100644
index 0000000..25155a7
--- /dev/null
+++ b/tests/setLanguage.test.ts
@@ -0,0 +1,320 @@
+import { setLanguage } from '../src/sheetsl';
+
+describe('setLanguage', () => {
+  beforeEach(() => {
+    // eslint-disable-next-line @typescript-eslint/no-empty-function
+    jest.spyOn(console, 'error').mockImplementation(() => {});
+  });
+  afterEach(() => {
+    jest.clearAllMocks();
+  });
+  it('should set the language', () => {
+    global.SpreadsheetApp = {
+      getUi: jest.fn(() => ({
+        Button: { OK: 'ok' },
+        ButtonSet: { OK_CANCEL: 'ok_cancel' },
+        prompt: jest
+          .fn()
+          .mockReturnValueOnce({
+            // prompt for source language
+            getSelectedButton: jest.fn(() => 'ok'),
+            getResponseText: jest.fn(() => 'EN'),
+          })
+          .mockReturnValueOnce({
+            // prompt for target language
+            getSelectedButton: jest.fn(() => 'ok'),
+            getResponseText: jest.fn(() => 'DE'),
+          }) as unknown as GoogleAppsScript.Base.PromptResponse,
+        alert: jest.fn(),
+      })),
+    } as unknown as GoogleAppsScript.Spreadsheet.SpreadsheetApp;
+    global.PropertiesService = {
+      getUserProperties: jest.fn(() => ({
+        getProperties: jest.fn(() => ({})), // No existing settings
+        getProperty: jest.fn(() => 'Sample-API-Key:fx'),
+        setProperties: jest.fn(),
+      })),
+    } as unknown as GoogleAppsScript.Properties.PropertiesService;
+    global.UrlFetchApp = {
+      fetch: jest
+        .fn()
+        .mockReturnValueOnce({
+          // deepLGetLanguages('source')
+          getContentText: jest.fn(() =>
+            JSON.stringify([
+              { language: 'EN', name: 'English' },
+              { language: 'DE', name: 'German' },
+            ]),
+          ),
+          getResponseCode: jest.fn(() => 200),
+        })
+        .mockReturnValueOnce({
+          // deepLGetLanguages('target')
+          getContentText: jest.fn(() =>
+            JSON.stringify([
+              { language: 'EN-US', name: 'English (US)' },
+              { language: 'DE', name: 'German' },
+            ]),
+          ),
+          getResponseCode: jest.fn(() => 200),
+        }),
+    } as unknown as GoogleAppsScript.URL_Fetch.UrlFetchApp;
+    setLanguage();
+    expect(global.UrlFetchApp.fetch).toHaveBeenCalledTimes(2);
+    expect(console.error).not.toHaveBeenCalled();
+  });
+  describe('should catch an error', () => {
+    afterEach(() => {
+      jest.clearAllMocks();
+    });
+    it('when the user cancels the language setup after notification of existing settings', () => {
+      global.SpreadsheetApp = {
+        getUi: jest.fn(() => ({
+          Button: { YES: 'yes' },
+          ButtonSet: { YES_NO: 'yes_no' },
+          prompt: jest.fn(),
+          alert: jest.fn(() => 'no'), // cancel the setup
+        })),
+      } as unknown as GoogleAppsScript.Spreadsheet.SpreadsheetApp;
+      global.PropertiesService = {
+        getUserProperties: jest.fn(() => ({
+          getProperties: jest.fn(() => ({
+            targetLocale: 'DE', // existing settings
+          })),
+        })),
+      } as unknown as GoogleAppsScript.Properties.PropertiesService;
+      global.UrlFetchApp = {
+        fetch: jest.fn(),
+      } as unknown as GoogleAppsScript.URL_Fetch.UrlFetchApp;
+      setLanguage();
+      expect(global.UrlFetchApp.fetch).not.toHaveBeenCalled();
+      expect(console.error).toHaveBeenCalledWith(
+        expect.stringMatching(
+          /^Error: \[SheetsL\] Canceled: Language setting has been canceled.\n/,
+        ),
+      );
+    });
+    it('when the user cancels the language setup after prompted to enter source locale', () => {
+      global.SpreadsheetApp = {
+        getUi: jest.fn(() => ({
+          Button: { OK: 'ok' },
+          ButtonSet: { OK_CANCEL: 'ok_cancel' },
+          prompt: jest.fn().mockReturnValueOnce({
+            // prompt for source language
+            getSelectedButton: jest.fn(() => 'cancel'), // cancel the setup after prompted to enter source locale
+            getResponseText: jest.fn(() => 'EN'),
+          }) as unknown as GoogleAppsScript.Base.PromptResponse,
+          alert: jest.fn(),
+        })),
+      } as unknown as GoogleAppsScript.Spreadsheet.SpreadsheetApp;
+      global.PropertiesService = {
+        getUserProperties: jest.fn(() => ({
+          getProperties: jest.fn(() => ({})), // No existing settings
+          getProperty: jest.fn(() => 'Sample-API-Key:fx'),
+          setProperties: jest.fn(),
+        })),
+      } as unknown as GoogleAppsScript.Properties.PropertiesService;
+      global.UrlFetchApp = {
+        fetch: jest
+          .fn()
+          .mockReturnValueOnce({
+            // deepLGetLanguages('source')
+            getContentText: jest.fn(() =>
+              JSON.stringify([
+                { language: 'EN', name: 'English' },
+                { language: 'DE', name: 'German' },
+              ]),
+            ),
+            getResponseCode: jest.fn(() => 200),
+          })
+          .mockReturnValueOnce({
+            // deepLGetLanguages('target')
+            getContentText: jest.fn(() =>
+              JSON.stringify([
+                { language: 'EN-US', name: 'English (US)' },
+                { language: 'DE', name: 'German' },
+              ]),
+            ),
+            getResponseCode: jest.fn(() => 200),
+          }),
+      } as unknown as GoogleAppsScript.URL_Fetch.UrlFetchApp;
+      setLanguage();
+      expect(global.UrlFetchApp.fetch).toHaveBeenCalledTimes(2);
+      expect(console.error).toHaveBeenCalledWith(
+        expect.stringMatching(
+          /^Error: \[SheetsL\] Canceled: Language setting has been canceled.\n/,
+        ),
+      );
+    });
+    it('when the user enters a value not included in the list of available source languages', () => {
+      global.SpreadsheetApp = {
+        getUi: jest.fn(() => ({
+          Button: { OK: 'ok' },
+          ButtonSet: { OK_CANCEL: 'ok_cancel' },
+          prompt: jest.fn().mockReturnValueOnce({
+            // prompt for source language
+            getSelectedButton: jest.fn(() => 'ok'),
+            getResponseText: jest.fn(() => 'JA'), // a source locale that is not included in the response value of deepLGetLanguages('source')
+          }) as unknown as GoogleAppsScript.Base.PromptResponse,
+          alert: jest.fn(),
+        })),
+      } as unknown as GoogleAppsScript.Spreadsheet.SpreadsheetApp;
+      global.PropertiesService = {
+        getUserProperties: jest.fn(() => ({
+          getProperties: jest.fn(() => ({})), // No existing settings
+          getProperty: jest.fn(() => 'Sample-API-Key:fx'),
+          setProperties: jest.fn(),
+        })),
+      } as unknown as GoogleAppsScript.Properties.PropertiesService;
+      global.UrlFetchApp = {
+        fetch: jest
+          .fn()
+          .mockReturnValueOnce({
+            // deepLGetLanguages('source')
+            getContentText: jest.fn(() =>
+              JSON.stringify([
+                { language: 'EN', name: 'English' },
+                { language: 'DE', name: 'German' },
+              ]),
+            ),
+            getResponseCode: jest.fn(() => 200),
+          })
+          .mockReturnValueOnce({
+            // deepLGetLanguages('target')
+            getContentText: jest.fn(() =>
+              JSON.stringify([
+                { language: 'EN-US', name: 'English (US)' },
+                { language: 'DE', name: 'German' },
+              ]),
+            ),
+            getResponseCode: jest.fn(() => 200),
+          }),
+      } as unknown as GoogleAppsScript.URL_Fetch.UrlFetchApp;
+      setLanguage();
+      expect(global.UrlFetchApp.fetch).toHaveBeenCalledTimes(2);
+      expect(console.error).toHaveBeenCalledWith(
+        expect.stringMatching(
+          /^Error: \[SheetsL\] Invalid Value \(JA\): Enter a valid value.\n/,
+        ),
+      );
+    });
+    it('when the user cancels the language setup after prompted to enter target locale', () => {
+      global.SpreadsheetApp = {
+        getUi: jest.fn(() => ({
+          Button: { OK: 'ok' },
+          ButtonSet: { OK_CANCEL: 'ok_cancel' },
+          prompt: jest
+            .fn()
+            .mockReturnValueOnce({
+              // prompt for source language
+              getSelectedButton: jest.fn(() => 'ok'),
+              getResponseText: jest.fn(() => 'EN'),
+            })
+            .mockReturnValueOnce({
+              // prompt for target language
+              getSelectedButton: jest.fn(() => 'cancel'), // cancel the setup after prompted to enter target locale
+              getResponseText: jest.fn(() => 'DE'),
+            }) as unknown as GoogleAppsScript.Base.PromptResponse,
+          alert: jest.fn(),
+        })),
+      } as unknown as GoogleAppsScript.Spreadsheet.SpreadsheetApp;
+      global.PropertiesService = {
+        getUserProperties: jest.fn(() => ({
+          getProperties: jest.fn(() => ({})), // No existing settings
+          getProperty: jest.fn(() => 'Sample-API-Key:fx'),
+          setProperties: jest.fn(),
+        })),
+      } as unknown as GoogleAppsScript.Properties.PropertiesService;
+      global.UrlFetchApp = {
+        fetch: jest
+          .fn()
+          .mockReturnValueOnce({
+            // deepLGetLanguages('source')
+            getContentText: jest.fn(() =>
+              JSON.stringify([
+                { language: 'EN', name: 'English' },
+                { language: 'DE', name: 'German' },
+              ]),
+            ),
+            getResponseCode: jest.fn(() => 200),
+          })
+          .mockReturnValueOnce({
+            // deepLGetLanguages('target')
+            getContentText: jest.fn(() =>
+              JSON.stringify([
+                { language: 'EN-US', name: 'English (US)' },
+                { language: 'DE', name: 'German' },
+              ]),
+            ),
+            getResponseCode: jest.fn(() => 200),
+          }),
+      } as unknown as GoogleAppsScript.URL_Fetch.UrlFetchApp;
+      setLanguage();
+      expect(global.UrlFetchApp.fetch).toHaveBeenCalledTimes(2);
+      expect(console.error).toHaveBeenCalledWith(
+        expect.stringMatching(
+          /^Error: \[SheetsL\] Canceled: Language setting has been canceled.\n/,
+        ),
+      );
+    });
+    it('when the user enters a value not included in the list of available source languages', () => {
+      global.SpreadsheetApp = {
+        getUi: jest.fn(() => ({
+          Button: { OK: 'ok' },
+          ButtonSet: { OK_CANCEL: 'ok_cancel' },
+          prompt: jest
+            .fn()
+            .mockReturnValueOnce({
+              // prompt for source language
+              getSelectedButton: jest.fn(() => 'ok'),
+              getResponseText: jest.fn(() => 'DE'),
+            })
+            .mockReturnValueOnce({
+              // prompt for target language
+              getSelectedButton: jest.fn(() => 'ok'),
+              getResponseText: jest.fn(() => 'JA'), // a target locale that is not included in the response value of deepLGetLanguages('target')
+            }) as unknown as GoogleAppsScript.Base.PromptResponse,
+          alert: jest.fn(),
+        })),
+      } as unknown as GoogleAppsScript.Spreadsheet.SpreadsheetApp;
+      global.PropertiesService = {
+        getUserProperties: jest.fn(() => ({
+          getProperties: jest.fn(() => ({})), // No existing settings
+          getProperty: jest.fn(() => 'Sample-API-Key:fx'),
+          setProperties: jest.fn(),
+        })),
+      } as unknown as GoogleAppsScript.Properties.PropertiesService;
+      global.UrlFetchApp = {
+        fetch: jest
+          .fn()
+          .mockReturnValueOnce({
+            // deepLGetLanguages('source')
+            getContentText: jest.fn(() =>
+              JSON.stringify([
+                { language: 'EN', name: 'English' },
+                { language: 'DE', name: 'German' },
+              ]),
+            ),
+            getResponseCode: jest.fn(() => 200),
+          })
+          .mockReturnValueOnce({
+            // deepLGetLanguages('target')
+            getContentText: jest.fn(() =>
+              JSON.stringify([
+                { language: 'EN-US', name: 'English (US)' },
+                { language: 'DE', name: 'German' },
+              ]),
+            ),
+            getResponseCode: jest.fn(() => 200),
+          }),
+      } as unknown as GoogleAppsScript.URL_Fetch.UrlFetchApp;
+      setLanguage();
+      expect(global.UrlFetchApp.fetch).toHaveBeenCalledTimes(2);
+      expect(console.error).toHaveBeenCalledWith(
+        expect.stringMatching(
+          /^Error: \[SheetsL\] Invalid Value \(JA\): Enter a valid value.\n/,
+        ),
+      );
+    });
+  });
+});