We maintain translations of the XStore Core plugin here.
If you want to contribute translated strings to your language register at 8theme.com and start translating. You can find a video tutorial on how to finish the translation here https://youtu.be/YjPNB5TAYfM?t=191
If you wish to start a new translation of the language that does not exist in the list you can submit a request here https://www.8theme.com/contact-us/ and choose the Suggestions & Feature Requests department.
We would be very grateful if you could share new translation files with us or finish the existing translations to help other users.
Keep in mind that if you translated the XStore core plugin but still see any missing strings make sure that you translated XStore Theme https://www.8theme.com/glotpress/projects/xstore/xstore-theme/