There is a difference between asking for language proficiency and looking for native speakers.
Within the EU one could argue that a job offer limited to native speakers is discriminatory UNLESS there is a plausible justification (e.g. language teachers, actors).
One has to admit, however, that for most "smaller" languages most people are pretty unlikely to have knowledge of the language unless they're native speakers. Who on earth would learn Finnish, Hungarian, Bulgarian, Dutch, Greek, etc. as a second language? (Not meaning to debase these languages in any way but they're just not popular as second languages.)