Most Latin adjectives related to names of countries and languages are of first and second declension: Latinus, Graecus, Anglicus… If I want to express that I speak in any such language, I will simply use the derived adverb: Latine/Graece/Anglice loquor.
But how does it work if the adjective belongs to the third declension? The list in the linked question contains, for example, Islandiensis, Melitensis, and Thailandiensis. If I want to express speaking or otherwise using these languages, do I use the adverb normally? Saying Islandiensiter/Melitensiter/Thailandiensiter loquor sounds weird. I have never seen such adjectives in such use, so I have no intuition. (I don't know whether the Romans ever had third declension adjectives together with lingua, so there may or may not be classical help.)
Is the -iter ending used under such circumstances? Is a first and second declension alternative always used instead (Islandice, Melitice etc.)? Or should I simply use the ablative like lingua Melitensi loquor?