The same problem for smj language code. It is possible to select the language but not to save the statement afterwards.

Note that sr-cyrl is a variant of sr, while smj is a normal language code. This could imply that there are two bugs, but I suspect that it is not.

If all codes that is non-supported by the localization fails, then that is perhaps an indication where the bug bites?

The language code sma works, it is Souther Sami Language (https://en.wikipedia.org/wiki/Southern_Sami_language) which is a very small language spoken by 500-2000 people.

This is a more complicated problem. We are using two sources for the languages. Frontend uses ULS because this is much more convenient for the user:

core/extensions/UniversalLanguageSelector/lib/jquery.uls/data/langdb.yaml

Backend uses a core feature:

core/languages/Names.php

These lists are different. Very, very different, actually. I can't say why, at the moment. So yes, we are aware that this situation clearly needs to be improved.

I think it is important to fix this for the mono-/multilingual text as it create a situation where official names in some official languages can't be given. This is the case for Norway, where we can't give the official name in some of the recognized minority languages.

This issue has nothing to do with the Norwegian languages, because they are not supported by either of the data source we have at hand.

https://github.com/wmde/ValueView/pull/144 makes the language lists used by the frontend and the backend match. That does not add support for the mentioned languages, but it at least makes behavior consistent.

https://github.com/wmde/ValueView/pull/144 is merged, so now this is about the language code not being accepted at all.

Another language code not currently accepted is lld (Ladin language, Q36202, spoken by some 30.000 people on the Dolomiti mountains in Italy)

It should be possible to allow the extra languages by adding them to $wgExtraLanguageNames.

Other requests I've seen:

Cape Verdean Creole (kea) at https://meta.wikimedia.org/wiki/Requests_for_new_languages/Wikidata_Cape_Verdean_Creole
Finnish Swedish (sv-fi) at https://www.wikidata.org/wiki/Wikidata:Contact_the_development_team/Archive/2015/05#Finland_Swedish_.28Q1461092.29
Korean (Hanja) (ko-kore?) at https://www.wikidata.org/wiki/Wikidata:Contact_the_development_team#Can_we_have_monolingual_text_in_Korean_in_Hanja_or_Hangul.3F
Koyukon (koy) at https://www.wikidata.org/wiki/Wikidata:Contact_the_development_team/Archive/2015/09#expand_UserLanguageCode_options
Lakota (lkt) at https://www.wikidata.org/wiki/Property_talk:P1559#Cannot_add_a_Lakota_name
Old High German (goh) at https://www.wikidata.org/wiki/Wikidata:Contact_the_development_team/Archive/2015/09#expand_UserLanguageCode_options

Talossan was also requested at https://meta.wikimedia.org/wiki/Requests_for_new_languages/Wikidata_Talossan

I'm really lacking an Etruscan support (ett). There's a good deal of ancient italian location/city-state that bears an Etruscan name or has an Etruscan etymology (Rome, to say the least…)…

This following bug will most likely be fixed when the following few lines are added to the Names.php file.
The lines that needs to be added are:

	'ett' => 'mechl Rasnal',	# Etruscan
	'fkv' => 'kvääni',	# Kven
	'goh' => 'diutisk',	# Old High German
	'kea' => 'Kabuverdianu',	# Cape Verdean Creole
	'koy' => 'Denaakkenaageʼ',	# Koyukon
	'lkt' => 'Lakȟótiyapi',	# Lakota
	'lld' => 'Ladin',	# Ladin
	'rmg' => 'tavringens rakripa',	# Traveller Norwegian
	'smj' => 'julevsámegiella',	# Lule Sami
	'tzl' => 'Talossan',	# Talossan

I do not know how to add those lines myself, so maybe somebody else can add this line for us all.