Well technically

a thread is not a subject, if you are looking for another word 'topic' might come closer (mind that english is not my native language).
A thread is the first post and all answers to that, thus forming a 'thread' of posts, posts linked together if you wish.
Many subject could be discussed in 1 thread (although that is not very likely), and many threads on 1 subject could exist.
You might also export/Download the MASTER LANGUAGE and then select only phrases, probably your board must be in debug mode to do so. You could then edit the xml-file (make sure you don't edit the phrase names, only content), and then upload it again as an alternative language. I would make some tests on a testboard before you start doing this.