Fix find language code

array_intersect() conservs keys, so sometimes
the first element hasn't 0 as key.

Fix PHP 8.1 issue, declaring the array before use.

Test case:
Browser returns ['en-US', 'en', 'fr-BE', 'fr']
Zed accepts ['en', 'fr']

Language is now 'en' by intersect,
not because 'en' is the fallback.

Ref. T108