There is no centralized or organized system. Basically, any user can decide any moment to make subtitles for any movie or episode.
In reality, translators often stick to one or a few tv shows and do a whole season. Recent and popular series are more likely to be translated.
You can request subtitles in the request section (the link "בקשה
" at the top of every page).
For as far as you did not do it yet
please do not request in this forum, it would be flooded in no-time.