In English, when referring to a sentence written in French, I could say "the French sentence" or "the sentence in French". English is not my first language, but I think those two variants can be used interchangeably. Does the same apply to Spanish, i.e. could I use both "la frase francesa" and "la frase en francés", or is one better/more idiomatic than the other?
Both can be correct.
That said, en francés is much better, mainly because when using the adjective form, it could be interpretted either as a phrase either rendered in the French language (en la lengua francesa) or as a phrase with its origin in France (del país francés) which may or may not be rendered in the French language:
If you believe RAE (Royal Academy), francesa as an adjective cannot mean French. Therefore "Literatura francesa de Quebec" would be a contradiction.
This means that according to RAE you should say “la frase en francés”.
Whenever I read “la Wikipedia inglesa” (14,300 times in Google) I agree with RAE, because to me that sentence means “Wikipedia from England”.