Well showing area that has only minority of some speakers as the colour of that language is quite misleading. Should be shaded or something
It depends. I think if both languages are exclusive to that area then yeah, flag it. If one is a larger language and the other a minority language exclusive to a region is fine to only show the minority one. Context solves the issue just fine.
You don’t know from context what the actual language situation there is with that method. For informative maps that’d be bad. That’s why shading makes more sense.
If your map is about where every language is spoken, then you shade where every language is spoken.
If your map is about how a word is said on different languages, then you place words for unique languages, context takes care of bilingual areas.
It seems pretty obvious, really.