Firefox supports as language settings Norwegian with the language code no. It also supports Bokmål with the language code nb and Nynorsk with the language code nn. When you set as preferred language no as first language and an other language code like nl as second language you get the HTTP header
According to https://www.iana.org/assignments/language-subtag-registry no (Norwegian) is a macro language with the languages nb (Norwegian Bokmål) and nn (Norwegian Nynorsk).
MediaWiki normally substitutes the language code no by nb.
ULS does not support the fallback from no to nb for the Accept-Language HTTP header and ignores no:
$ curl -L -H 'Accept-Language: no,nl;q=0.5' 'http://localhost/mediawiki/' 2> /dev/null | head -2
<!DOCTYPE html> <html class="client-nojs" lang="nl" dir="ltr">
Expected result is lang="nb" or lang="no".
$ curl -L -H 'Accept-Language: no,nl;q=0.5' 'http://localhost/mediawiki/mw-config/' 2> /dev/null | head -2
<!DOCTYPE html> <html dir="ltr" lang="nl">
The language code bh has the same problem like no.
This bug is related to T183295.