When do you choose to translate compliance training into languages other than US English? Are there principles you apply when it isn't a legal requirement?

The simple principle applied here is that anything brought published through your management system should be clearly managed and made available for its intended audience to have impact. That said, if making training available in other languages other then the standard company language makes it more effective, it should be considered. 

We do it based on volume of language speakers. We have a training contract for a set amount of languages and we select the languages based on highest number of online users that can be reached in their language.