Can you learn a language just from video games?
Probably not, and we don't recommend trying, especially for advanced languages like Japanese. It's a great way to break up your studying, though. (In other words, consider this website a supplement for classes, textbooks, etc., not a replacement for them.)
 What level of Japanese [or whatever language] do I need to have already?
It depends on the game. Don't try to tackle something huge like an RPG with only a very basic level of Japanese; you will find it overwhelming and boring. Hence, the more text a game has, the higher level of Japanese we will tend to assume. Something like Super Mario Bros. 3 can be played with almost any level of Japanese because it has so little text, though be warned that even short texts can use vocabulary and grammar that's quite advanced. The good news is, that advanced vocabulary and grammar will probably be explained for those short texts; there is no such guarantee for longer texts.
 What's the difference between a script dump and a transcript?
A script dump was ripped directly from the game data (typically through ROM hacking). A transcript was made by hand by playing through the game and writing down the text or dialogue as you see or hear it. Transcription is very time-consuming for long games with a lot of text, but may be more expedient for short games with little text. Transcription also requires less technical skill.
Note: be very careful when transcribing, especially if the text is in a language like Japanese, where typos can be very hard to spot! Working from text dumps ensures that you won't get thrown off by a bad transcription.
 What are TBL files?
If you don't know what they are, you probably don't need them. (They're for ROM hacking; they allow the game's text to be decoded into a human-readable format, or encoded back into a format the game will understand.)
 Why does this site concern itself with ROM hacking so much?
Because ROM hacking is often the only practical way to extract a game's script. Thus, we find it prudent to provide tools and other relevant resources to teach others how it's done.