What high-level problem are you trying to solve?
When you do a multi-lingual survey, it's important to know what language the user selected because it could affect the analysis. For example, a bad translation in a choice list.
Any ideas on how ODK could help you solve it?
Currently, you can put a question in form asking the data collector which language they are using, but it's not a guarantee that they are actually using that language.
We could add form language as metadata (like deviceID), but because language can be changed at any time during the form filling session "chosen language", we'd have to be very precise about what it means (e.g., language on the last question).
I think the likely best option is to add it to the form audit log (e.g., form language changed). It won't be as easily accessible as metadata, but at least it'll be available for use.