There is a slightly newer version of that up on filetrip (preview 2), I have a newer version in the works but I am mainly adding the odd paragraph/chapter here and there.
That looks like the start of the table, if the font from 2 is anything to go by then it is incomplete but probably enough for those tweaking English wording to get started.
I know nothing of Lithuanian so everything I now know comes from skimming http://www.omniglot.com/writing/lithuanian.htm
2. Is that the font from the game or just a random one you found somewhere? If it is the font from the game then you might be in for an easier time.
By the looks of things it is not a massively different character set and just has an extra 18 characters (both cases) and no Q, W or X. This is good as you probably do not have to do a massive hack like a 16 bit conversion or something.
If that is the font then chances are the less than complete table above and the far more full font in the image will combine to allow you an option. It will not allow you to easily have it so the standard unicode encodings for the characters in question will be present but you will have them on some other encoding -- this is fine as tables can be made to go both ways and encode text as well (very few hackers will redo an entire encoding if they can get the characters they need into the game). The nosinė stuff might pose a problem as most Japanese games are not built to handle things that go below the lines (see other characters like y,p and g.
3. Assuming you have the screen real estate/text buffer available then we get to cover basic GBA pointers.
Games do not know where things start or finish and trying to find that out is difficult for games so they do not tend to do it. Instead they will have long lists/databases of numbers that say where things can be found. If you make a section longer you get to update this list/database and any subsequent ones you might have changed.
The GBA ROM is seen in memory so for the most part the pointers will look to memory . This is not mandatory though (here is something from another Dragon Ball Z game I was playing with a few weeks back http://www.romhacking.net/forum/index.php/topic,16601.msg244683.html#msg244683
4. Just because a compression tool can not find compression does not mean there is none -- the GBA has seen custom compression as indeed that link to the other Dragon Ball Z game was dealing with.
5. Unlikely -- the table you linked lines with ASCII and so even a basic hex editor would display a halfway readable script. Indeed I just did an ascii strings search and it spat out loads of readable text from my copy of Buu's fury.