Assuming SRTs don't support sub-documents, contentHash should be safe (ie unique).  ContentHash is generally a good place to start.

Note that it does mean that two identical strings will only be translated once (and thus translated to the same thing), but that is usually fine.  If you think that might be a problem, you could index strings by their timestamps.  At a guess, the timestamp would probably map to the textUnitId, but we'd have to check the Okapi source to be certain.