Use Javascript all the way... You can get the locale from a combination of navigator.language for iOS, the navigator.userAgent for Android, and various properties on WP7 (navigator.browserLanguage, systemLanguage, and userLanguage)
Define your translation matrix, set up some convenience functions (I use __T("text"), __N(number), __C(currency), etc.) and you should be good to go. I also take $() and extend it a bit to include the locale so that I can have use HTML blocks without having to do a lot of nutty innerHTML work when dealing with paragraphs, headings, and more. For example, I might have a DIV called "welcome_enus" and one called "welcome_eses" and my $Localized("welcome") returns either the _enus or _eses version appropriately.
Or use a pre-built library. There should be several out there.
Unless you're intending to stay iOS only, I would add one more reason to stay with Javascript for this purpose: it stays multi-platform compatible, whereas you'd have to redo the work for each platform if you implemented the localization natively.