GN-4808: Fix language detection for first time users

[dkozickis]

GN-4808: Fix language detection for users that are coming to GN for the first time

Overview

There's a chance that matchUserLocaleToSupportedLocale will return undefined value, calling toLowerCase will then throw an error, this might happen when application is loading for the first time for the user, and there's nothing yet stored in the browser for the user.

connected issues and PRs:

• Caused by https://binnenland.atlassian.net/browse/GN-4772
• Ticket - https://binnenland.atlassian.net/browse/GN-4808

Setup

1. Checkout
2. ember s --proxy https://reglementairebijlagen.lblod.info

How to test/reproduce

1. Try to open in incognito tab

Challenges/uncertainties

There's no TS in this project, without it's help I've missed this possibility.

Checks PR readiness

• UI: works on smaller screen sizes
• UI: feedback for any loading/error states
• Check cancel/go-back flows
• Check database state correct when deleting/updating (especially regarding relationships)
• changelog
• npm lint
• no new deprecations