Depends on the kind of apps. Japan and China are huge markets, but a western oriented app may not work there. While that same app can work when localized for France, Germany etc
These are the major languages I would look at
1. Spanish
2. Portugese
3. German
4. French
5. Chinese
6. Japanese
I've used gengo.com, icanlocalize.com, and also random sources - sometimes users help, our head of support speaks French, and our intern speaks Mandarin.
These are the major languages I would look at 1. Spanish 2. Portugese 3. German 4. French 5. Chinese 6. Japanese