fix: force languages in cypress browser for tests (#8049)

To fix this we had to create a free trial account on cypress and enable
the recording of the test. That way we found out the issue was with a
locale:
![image
(38)](https://github.com/user-attachments/assets/db2fad23-6fec-47c0-8c6f-a93f3e4e4c4c)

Probably, this works well locally because our local machines do have a
default locale, but probably we don't have one when running in CI, and
millify library is causing the tests to fail specifically at this line:
https://github.com/Unleash/unleash/blob/363911c4a1ce94dd467a950ec84ccdc9a3ec7d75/frontend/src/component/common/AvatarGroup/AvatarGroup.tsx#L89
(validated
[here](https://github.com/Unleash/unleash/pull/8040/files#diff-afc857890da2221bd34feed0ff45dd7745ff32fb0b27055214cbe69896d5311dL89)).

Unfortunately, upgrading millify didn't help, but downgrading to v5
(which doesn't support locales), solve the issue at the cost of not
having the up-to-date library:
#8048

I believe the issue is related to this locale `c` reported here:
cypress-io/cypress#7890 (comment)
because only after overriding the languages this worked

.github/workflows/build_frontend_prs.yml

@@ -4,6 +4,7 @@ on:
   pull_request:
     paths:
       - frontend/**
+      - .github/workflows/**frontend**
 
 jobs:
   build:

frontend/cypress/global.d.ts

@@ -55,7 +55,7 @@ declare namespace Cypress {
         // STRATEGY
         addUserIdStrategyToFeature_UI(
             featureName: string,
-            projectName?: string,
+            projectName: string,
         ): Chainable;
         addFlexibleRolloutStrategyToFeature_UI(
             options: AddFlexibleRolloutStrategyOptions,

frontend/cypress/integration/feature/feature.spec.ts

@@ -60,17 +60,10 @@ describe('feature', () => {
         cy.addFlexibleRolloutStrategyToFeature_UI({
             featureToggleName,
             project: projectName,
-        }).then(() => {
-            cy.updateFlexibleRolloutStrategy_UI(
-                featureToggleName,
-                projectName,
-            ).then(() =>
-                cy.deleteFeatureStrategy_UI(
-                    featureToggleName,
-                    false,
-                    projectName,
-                ),
-            );
         });
+
+        cy.updateFlexibleRolloutStrategy_UI(featureToggleName, projectName);
+
+        cy.deleteFeatureStrategy_UI(featureToggleName, false, projectName);
     });
 });

frontend/cypress/support/commands.ts

@@ -26,6 +26,10 @@ import {
     //@ts-ignore
 } from './API';
 
+Cypress.on('window:before:load', (window) => {
+    Object.defineProperty(window.navigator, 'language', { value: 'en' });
+    Object.defineProperty(window.navigator, 'languages', { value: ['en'] });
+});
 Cypress.Commands.add('runBefore', runBefore);
 Cypress.Commands.add('login_UI', login_UI);
 Cypress.Commands.add('createSegment_UI', createSegment_UI);