Error banners are not localized for non-English device languages

Description

When device/system language is set to a non-English language (e.g. Slovak), the error banner shown after attempting to log in with an invalid credentials is displayed in English, e.g. "Invalid username or password". This suggests the string is hard-coded or missing from the translation resources.

Environment
PROD

Application (+ app version, build, operating system)

• Actual Android app version: 2025.6.0
• Actual iOS app version: 2025.6.0
• iOS 18.5

Steps to reproduce

1. Set device system language to Slovak (or any supported non-English language).
2. Launch Mergin Maps mobile app (ensure logged out).
3. Navigate to the Login screen.
4. Enter a valid existing username.
5. Enter an incorrect password.
6. Tap Sign in.
7. Observe the error banner text.
8. Enter valid email and leave the password empty -> error banner is in ENG

Actual results
Error banner text is shown in English despite device language being non-English.

Expected results
The banner should display the translated string for the current device/app language. If the translation key is missing, a fallback/missing key log should appear and the key should be added to localization files. English should not appear silently when a translation is expected to exist.

Screenshots or logs

Conclusion
Created by GitHub Copilot and supervised by a human test analyst.

No files, tests, or entry points are named; start by locating the login error-banner handling and the localization resources used by the Android and iOS apps. Reproduce the invalid-password and empty-password cases with a supported non-English device language, then verify that both banners use translated strings rather than silently displaying English.