Well, the question you need to ask yourself: Do I want to play on an EU or US server? Because Odin is EU and Sargatanas is US.

I am on Odin and I always find it to be very active, and English is indeed the most commonly used language. There are some German/French speaking FCs but they mostly keep to theirselves and 90% of them still speak/understand English (be it very little, but enough to converse with them). The PF on Odin is always very full (Especially during peak hours, 20+ parties on there) and there are alot of FCs, from very casual to quite hardcore. The only thing I dislike about Odin is the (extremely) unstable marketprices. But that might be the case on every server...