QMAPS-2113 use qwant's "l" laguage cookie

[xem]

Description

• Use Qwant's "l=..." cookie (if possible) as the interface language of Qwant Maps
• Fallback on the first available/useable language in the accepted-languages browser header (same as before)
• Fallback on Qwant Maps default language if nothing fits (same as before)

Why

Better coherence with Qwant.com settings

[bbecquet]

@xem
From your last commits, it looks like your prettier config doesn't work anymore :/
It should be entirely automatic, saving you from the pain of doing that by hand. We'll find how to restore it.