At the top of the Weblate home page, click “Languages” and select “Explore all languages”. You will see a list of projects currently available for translation. Their names are self-explanatory. To localize LibreOffice, you'll have to work on projects called LibreOffice master – UI, LibreOffice master – Help, LibreOffice x.x.x – UI and LibreOffice x.x.x – Help. In general, if you're just starting, it probably makes most sense to work on master, but if you want to fix some nasty L10n bug in the current version of LibO, you can do that in the branch too.