If your creating apps with AppBuilder (either the embedded one in ArcGIS Online, or the Developer edition), you can make use of a URL parameter to change the language of the app.
To switch app language, use locale=. These language
codes are supported:
If you are building custom widgets for AppBuilder, you will have to factor in other languages in your code.
I believe the language URL parameter might also work on some of the other templates applications as well.
For an ArcGIS Online for Organisation, the Admin can set the language for the site. However, as a named user, if your browser uses a different language, you will see that language. You can also override the organisations language you see by updating your individual profile.
Hope that helps.