The whole site interface, including the tour, is in English, so I guess it is reasonable to expect that people use either English to communicate — or Japanese, which is the topic of the site.
I think it doesn't make sense to keep any contributions (comments, answers, questions) in other languages, since they probably cannot be understood — and thus also not moderated — by the majority of the community. We're always offering to help users having trouble formulating their question in English to edit their question to make it more clear.
The basic content of the tour is a fixed template, but in principle we can change parts of it. For example this paragraph appearing at the top of the tour:
Japanese Language Stack Exchange is a question and answer site for students, teachers, and linguists wanting to discuss the finer points of the Japanese language. It's built and run by you as part of the Stack Exchange network of Q&A sites. With your help, we're working together to build a library of detailed answers to every question about the Japanese language.
We could add a sentence:
Questions and answers can be written in Japanese and/or English.