1. Fork Repository
2. Create new Branch
3. Create folder for your language (values-xx, where xx is the two letter language code)
4. Create strings.xml and googleplay.txt in this folder
5. Copy content of values/strings.xml into this file and translate every string
6. Copy content of values/googleplay.txt into your file and translate every string
7. Create pull request and I'll accepp your translation

All these things can be done in the GitHub web interface, you only need an account (which is free). To create folder, click create file, type folder name followed by '/' to go down into a subfolder.