To translate Pleroma, add your language to [src/i18n/messages.js](https://git.pleroma.social/pleroma/pleroma-fe/blob/develop/src/i18n/messages.js). Pleroma will set your language by your browser locale, but you can temporarily force it in the code by changing the locale in main.js.
You don't need to build Pleroma yourself. Check out https://git.pleroma.social/pleroma/pleroma-fe/wikis/dual-boot-with-qvitter to see how to run Pleroma and Qvitter at the same time.