Population doesn't appear to be faked, but that's just my opinion. I believe the main language of the server is French, honestly. If not the most common, it's likely second. English, Spanish, French, are the languages I see most, with some German and Portuguese here and there. Probably more that I don't recognize or misrecognize. There are cross faction "world" channels for different languages, so when in world_en channel, people are speaking English.
Just about everything is cross-faction capable. Cross faction parties, mailing, dungeon finder and raid finder are as well, and guilds (at least I was told guilds were, haven't tested). Do be careful, healing cross faction is listed as against the rules, if it's done outside of a dungeon or raid. A lot of the PvP and PvE content are working. Some scripting is missing, but what I've seen so far (up to Outland), dungeon bosses can be fought and dungeons can be completed, with only minor bugs. (like the final boss of Wailing Caverns, Mutanus the Devourer being spawned at the very start of the dungeon, or the Foe Reaper 5000 boss in Deadmines only standing still when using Overdrive - if something seems broken, give it a little time, the script may very well be delayed like the Wailing Cavern's boss Helix Gearbreaker taking upwards of a minute to start phase 2 after the ogre dies).
Some quests while leveling might not be able to be completed, and some may have alternate ways of completing them. (e.g. a horde quest has you use a bomb item on two portals, but it doesn't work. Instead there are two custom NPCs at the portals that you just kill for quest credit). I have not found any truly 'gamebreaking' bugs yet, either with classes, skills, or content - but again, my highest level on the MoP server is in the 60s, so I haven't seen everything yet. Dunno about Isle of Thunder.
The realmlist works fine. The launcher just allows you to auto login, not having to type in your password every time.
I will note, one of the most annoying bugs is a combat bug, where you get stuck in combat randomly, preventing flight paths and even logging out. Solutions include entering/exiting a dungeon instance, or Alt+F4 (which using the launcher helps get in faster, not needing to retype password).
In general, I've played on servers where the devs are dedicated to a single expansion, yet are still far more buggy than Firestorm who is working on five at the same time. They do pretty well to make the game playable.