Some languages cannot be selected as a native language or a target language.
This is likely because many translators have already registered for working with these languages. We currently don't have enough assignments available given the number of excellent translators working with these language pairs. You cannot register for a different native language, because we work based on the principle that a person only has one native language. We expect that these languages will not be available for a year or more.
Also, we don't provide every language pair that exists in this world. If you want to know for which language pairs you can register, please take a look at this overview. If you navigate to 'Choose source language', you can open the menu to see the languages we provide.