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:
- la frase española: maybe in Spanish, maybe a Spanish saying
- la frase en español: definitely in Spanish (may or may not be from Spain)
- la frase de España: definitely from Spain (may or may not be in Spanish)
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”.
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An alternative could also be to use "la Wikipedia anglófona". Note, however, that the RAE doesn't necessarily encapsulate every single use especially when meaning my be transparent. The RAE itself published a book titled La primera versión castellana de Eneida, where clearly castellana refers to the language, though like francés, español, or inglés, there exists not adjectival definition similar to "perteneciente a la lengua castellana". Context will rule everything. – user0721090601 Jul 24 '14 at 23:53