I've tried to contribute to new languages, but they all have "join us" button that subscribes to a generic Mozilla mailing list, and the Voice team never sent any instructions how to contribute! No wonder these are stuck unfinished for years.
The page itself has only very few links. Randomly clicking through it logged in would have a high chance hitting one of the links that would either lead you directly to one of the links where you can CONTRIBUTE: https://voice.mozilla.org/de/speak, your dashboard where you can also find links to the CONTRIBUTE part or just the huge microphone symbol.
The actual method to contribute to them is finding Mozilla's internal translation tool, proposing translations, then finding a Mozillian to approve them. Then there's an obscure webapp for sentence submission and voting, but it doesn't give any feedback whether sentences you've submitted were accepted or went to /dev/null.
(you see people struggling with easiest things, because you assume people struggle with easiest things, rather than that you've misunderstood the problem.)
I've submitted 200 sentences, and none of the progress counters shown to me increased by 200, so I assumed they were lost, and gave up.