Probably the only thing you can do is built your own widget and connect that via the backend to the feedback API. I've also got an open request for making the widget responsive, so far no luck with this request on Mendix's behalf...
The default feedback widget is currently indeed not translatable. Mitchel's suggestion of using the Feedback API is a good one. The API is implemented as a webservice, allowing you to handle and submit feedback directly from your Mendix-application. You can find the documentation here: https://world.mendix.com/display/apidocs/Feedback+API.
would be great if someone makes this and put it in the appstore :) Preferably from Mendix itself so there is 100% sure there is no malicious code in it