As for why Orlesians never speak actual French I assume it's because 1) They don't want to force people to read subtitles when playing the game 2) Finding enough native speakers as voice actors would probably be a pain (unless I'm mistaken Leliana's voice actress is the only actual French person voicing an Orlesian). Also once you introduce French as Orlesian you'd probably need to do the same for the other languages like Antivan and such, which just multiplies all the previous problems.
They should do that though. When you have only characters of a particular language in a scene, or you have private asides between such characters, they should speak their own language. Think about how cool that is to see (and hear). It adds life and interest to a setting. The language barriers that exist become interesting story elements. Language itself is interesting all on its own.
Heck, they did that already for a few scenes, Fenris speaking qunlat to the arishok and the dwarven tavern song in DA:O. I don't even like Fenris, but I loved that scene. And the elves fall in and out of their language a good bit. The languages are one of the coolest things about the setting. It'd be nice to make them a little more prominent. Technically, you could even take lines from different localisations for ambient conversations in a given setting.
As for french speakers in DA, I believe Isolde's VA was also a native speaker. I don't know about any of the rest. But honestly, they could hire Quebeci actors to fill those roles, if you're going to actually be speaking. It's one province over. Spanish you can find in abundance even near the US sound studios they use. A lot of modern American actors, or people in any field, grew up in bilingual homes or around a lot of spanish, especially in the south. Fluency in spanish is pretty high. The big hubs are really international cities. Draw from the theater community. When you start casting for german or especially latin and the fantasy languages they use, it's tougher to find actors that can do that well I'm sure. But a lot of that has the benefit of limited expectations from listeners where there's no real world analogue or experience.
I understand the concerns about dividing an actor's focus. I still think those languages should be sprinkled in there in the appropriate settings and scenes. The problem, to me, is more about when they play off foreign accented characters for comedic effect, but that's really a writing issue.