[FEATURE] Edit preferences - Change language (Thai / English)

## Features
- Change language (Thai / English)

## Acceptance Criteria
**GIVEN:** the user is on the Settings page  
**WHEN:** the user switches the language to Thai or English  
**THEN:** the system updates all UI text to the selected language  
**AND:** when available, course descriptions are displayed in the selected language  
**AND:** the language preference is saved to the user’s account