Dual booting's pretty much your only option short of one of you picking up the other language.

Can't say for sure, but it might be possible to get Mozilla to have everything show up in Hebrew on an English OS. IIRC, Mozilla uses it's own internationalization code so it will work on other platforms, meaning it tends to bypass anything in the OS. I'd say it's at least worth exploring. If it doesn't work, you're out some time and nothing more.