SubSync internalization is done with gettext package, using standarised file formats.
Translation files are located in subsync/locale/[LANG]/LC_MESSAGES directories, containing two files: messages.po and messages.mo. First one is text file with translations from which second one (binary) is generated.
In order to create new translation, just copy one of existing messages.po, put it in directory according to language 2-letter code, edit po file e.g. with Poedit and generate messages.mo from it.
Language codes are stored in languages.py. Check if your language is present on languages2to3. If not, you may have to add it manually here and also to languages dictionary.
If you managed to translate SubSync to different language, please create pull request, or share messages.po with me.