I'm thinking of using this plugin (And thus needing translator api) to prevent people from speaking languages other than English in global chat. But how is it performing? I guess it can be pretty heavy or laggy if it has to check every single chat message online before it allows it to be posted in server chat? So can these two plugins be laggy?

Define laggy? You are calling out for a webrequest for each person, so it will not be instant, but it shouldn't really have much of any impact on the performance.