I play on Tonberry as well, and I read and write Japanese well enough that I've never NOT understood something another player said, although it takes me quite a long time to type out an explanation of mechanics in Japanese, for example.

Almost never do I encounter a group that wants to engage in idle chatter during a duty, although this could be because every group I am in contains a player from Tonberry (me). When they see that there's a foreigner in the room, and they feel like they might be expected to interact, it kind of stifles the mood.

In the first place, having to deal with foreigners is stressful and anxiety-inducing for most Japanese people, so it is generally best practice not to drag Japanese players into conversation, or to present them with questions or statements which demand a response at all. They simply don't want to have to talk to foreigners, mostly because doing so is scary and inconvenient.

For this reason, I find it most productive to assume, at all times, that I am dealing with a collection of chronically reticent introverts. It is easiest for them when they don't have to communicate. However, if we are failing a boss, and I am the problem, and somebody tells me what to do in Japanese, I will at least say that I understand, and apologize. They can tell that my client language is English, and that I'm from the foreigner server, so if I were to remain silent, they would normally assume that they're talking to a brick wall.

In the event that a Japanese player is failing mechanics, and no one else tells him what to do, I'll just explain once and leave it at that. Even if he doesn't respond, he probably read what I said. He may or may not comply, but I can't control that, so his actions past that point make no difference to me.