Obviously one great solution is to lobby phone operating systems to support these languages. My rather aged Android phone has a long list of languages which includes quite a few minority choices (several types of English). I've seen more recent installations where the choice of languages in English ..... only. If people are going to take the time to learn and use Welsh, Catalan or even Cornish then it would seem a small effort by comparison to allow them to load up language support for their choice.
Meanwhile ... see Terence's questions