If you take a look at the following WikiTree Help link...
... you'll notice the statement "The place name suggestions will not be perfect. The language selection is limited to two dozen modern languages. Use your best judgement and feel free to ignore the suggestions.".
So if the native language is not listed, you have only 2 options. You can either leave it blank OR select "Other".
You can always provide the Native Language information within the Biological Information that will help provide clarity (as I don't believe that it has to be elaborate in on itself).
You can also add a Sticker that's outlined on the following WikiTree Help link...
... That you can customize/specify which language the individual is a native speaker of.
For example, the following will reflect that the individual is a native speaker of English...
... Now just replace the en with af and it'll reflect that the individual is a native speaker of Afrikaans...
... And the language of the words "This user is a native speaker of English" will reflect the Afrikaans language "Hierdie gebruiker se moedertaal is Afrikaans".
I hope that I had explained this okay via text and hopefully your happy with the various options available to you.