Unless you just want to tell the complainant that there's nothing you can do, you really have to attempt to get a translation of the text.
In the first instance I would use machine translation just to see if the complaint was justified. While the results aren't guaranteed they should give you a good sense of what's being said. I agree that you shouldn't sanction someone based on the results, but it should give you a good idea of whether you want to proceed with the complaint or not.
At this point, if you want to take things further, you would have to find someone else who knows the language to confirm the translation - there might be a forum or Q&A site where you can ask if the machine translation is accurate for example - or you have to simply tell the complainant that there's nothing more you can do as you don't have the resources.
This is likely to leave them upset and will probably exacerbate the situation as they feel they have to take matters into their own hands to resolve their grievance.
In the longer term, you might want to consider making a rule that all in game conversations have to be in English and that any message not in English will be deleted. You might have to institute a flagging system so that people can bring these to your attention.