[FEATURE] [FRONTEND] Persist language selection

[0xMurage]

Describe the user story:

Currently, when a user selects the Swahili language, the current page text is translated as expected until they navigate to a different path/route. Selected language should persist across route navigation and page reloads (on the same browser).

~~If the user is logged in, selected language should persist across sessions.~~

Tasks (where applicable)

• [ ]

Tests (where applicable)

• [ ] User can change the app language
• [ ] Selected language is persisted across routes
• [ ] ~~Last selected language is persisted across user sessions~~

Acceptance Criteria

• [ ] Selected language is persisted across routes on same browser (for both logged in and anonymous users).