[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).