I am developing a bot to synch ISSN data [1] using WDTK. Creating a label/alias with the language code 'mul' (to indicate a multilingual value), or 'mis' (to indicate the language is unknown) does not work and returns an Exception from the wikimedia API ("[not-recognized-language] The supplied language code was not recognized"). These 2 codes are however documented at https://www.wikidata.org/wiki/Help:Monolingual_text_languages and listed at https://www.wikidata.org/wiki/Help:Wikimedia_language_codes/lists/all

I also cannot use these 2 codes when editing through the human interface.
I do however find items in Wikidata with 'mul' language codes.

Am I doing something wrong, did I misunderstand something, or is there a true problem with these codes ?

Also opened ticket in WDTK issue tracker : https://github.com/Wikidata/Wikidata-Toolkit/issues/509

Thanks
Thomas

[1] : ISSN Bot : https://www.wikidata.org/wiki/Wikidata_talk:WikiProject_Periodicals#Data_donation_from_ISSN_Register_(Followup)