Any of the designated "japanese" servers (meaning, the data center is in japan). Otherwise the Legacy servers have a decent mix.
You used to be able to pick anything without having to list a data center, and thus they held anyone who liked the server name, now you have to pick a data center and there's very VERY few truly mixed servers anymore because of it.
On top of that, a lot of 1.0 players got their servers merged and then re-designated as either a JP server or a NA server. I used to be on Selbina (because of the FFXI community), they removed that and merged everyone with... idk, some other one. I think that one is Legacy now and NA. Others got the JP marker on it. If you can trace back which those were, those are the servers with the highest mix of English/Japanese players.
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[XI] Surivere of Valefor
[XIV] Sir Surian Bedivere of Behemoth
http://na.finalfantasyxiv.com/lodestone/character/2401553/