Waning Moon Conrad

Word lists can be dangerous. Taking clarus for clear which is all the list gives you, you might not know that you're only getting the masculine, singular nominative form.

There are thirty forms of each noun and adjective. Three genders, five cases, (not including vocative) and singular/plural. Some of them double up in some declensions, for example, ablative and dative plural are always the same.

And videlicet means clearly, evidently, of course, so we shouldn't trust online translators either.

Solitary Man

You could see clarus in lingua, or clara. But it radically changes things. More often, you'd see the ablative or genitive of respect/characteristic, that is, more literally, [he is] clear with respect to his language. What is clear in language and in mind? Or who? To avoid ambiguity, I'd recommend the genitive:

Clarus linguae, clarus mentis.

Now, the phrase also has a second meaning to it - it does not only mean "clear in language, clear in mind", but also "distinguished in language, distinguished in mind", which is a very common usage for clarus (especially in the forms of praeclarus or clarissimus). If you don't mind the double meaning, by all means keep it. If you abhor the double meaning, evidens, candidus, apertus, and planus would clear up that confusion (no one should confuse "brightly colored" for candidus in the phrase).

PS: Also, if you're referring to activities rather than qualities, i.e. "he speaks clear[ly] in language, he thinks clear[ly] in mind", then you'd need adverbs. But contrary to Waning Moon Conrad, you would not do "clara in lingua" for the reason of making clarus agree with lingua. You don't take adjectives which modify something or someone else and have them agree with the nouns in prepositional or oblique phrases. He is clear in language will be clarus linguae not clara in lingua.

Emil

Wow. Many thanks for the help!

I think I'll settle on: Clarus linguae, clarus mentis

Waning Moon Conrad

Good idea. I think Solitary Man is right and the genitive of characteristic is the best choice.