What are some of the considerations needed when adding multilingual support for a .NET application designed to run in Windows CE (5)?
If it's only the app that needs localization, then the considerations are the same as with any app. All string should come from a resource file. Be aware that there is no such thing as Thread.CurrentCulture, so you have to track the locale for the app manually.
The use of resource files is probably the way to go. I'm actually working on a .NET mobile app as well and found this tutorial series very helpful:
http://www.dotnetfordevices.com/articles/50.html
It not only covers some basics of localization, but also an idea of how to target both the Standard and Professional platforms of Windows Mobile 6 (which I think could also work for version 5). The only real difference between Standard and Professional as far as I know is touchscreen, so I think you could really benefit from consolidation of your code base as much as possible.