One way to deal with the problem of people communicating in a language the project does not understand would be to let repo owners set one or two languages for the repo. When users on GitHub en Español see an input box on their repo, it would be labeled: "Este proyecto sólo acepta contribuciones en inglés o alemán." It won't solve the problem completely, but it would at least make expectations clear.
My concern with this obvious suggestion for the problem is that GitHub has done extremely little to improve the Issue system, Pull Requests aside where merging and continuous integration has been improved.
Imagine being the proprietor of Twitter Bootstrap and having to deal with this. What is an acceptable rate of admission of issue creators who are not proficient in English?
If 20% of "international" users are undeterred by the warnings and labels, the repo owners should still receive a free coupon for Prozac from GitHub. :)
