For language_not_allowed the selected language of your comment must be among allowed languages on the community/instance your commenting on. This whole Language thing can be a bit confusing depending on the settings of your instance.

I think the jerboa client (and others?) does NOT let you select the language of your comment, so comment once from the Web interface, make sure you select language (English). After I did that I can now comment from Jerboa, so I guess my instance remembers the settings.

Also go in to your profile settings and tick "undetermined" language setting, in addition to English and whatever else you use

Here are some links I had a similar issue: good explanation, and issue solve by my Instance admin: https://reddthat.com/post/1976

Ferdig.uk changing from English only to also undetermined https://feddit.uk/post/37778