A text-input bug affecting many Arabic speakers was discovered when we were about make Firefox 3.5 release candidates, and we chose to include a fix in Firefox 3.5.1 rather than delay Firefox 3.5.
The reason we found out about these bugs late isn't a lack of non-English beta users. 1.8% of English Firefox users are beta users, and 1.6% of Russian Firefox users are beta users. Arabic beta use is much higher, at 3.5%, perhaps because Firefox 3 was such a large improvement over Firefox 2 for rendering Arabic text.
Instead, the reason may be that people who don't know English can't report bugs easily. They can't even search Bugzilla to find out whether their bug is known. They went out of their way to try betas, but beta-testing alone does not keep major problems from making their way into a release.
How can we enable the rest of the world to participate in reporting bugs, or at least ensure that we find out about the most frequent problems in other languages?