Simple way. If there are characters or punctuation in the game that are not used or that you can work around* then overwrite those with the relevant characters.
*take your finished script. If it turns out there are no uses of say the z character (or maybe Z as upper and lower case are different) then despite it being a character in the language you can still afford to overwrite it. If there is say only one use of it then maybe think if you can edit your translation to use a word without z.
If you can find and decode the font this is usually about as easy as playing with an editor. Can get a bit more tricky in the case of variable width fonts but there you either find one of suitable size or see if the game uses simple size values.
This naturally works better for Japanese where a given game might have thousands of characters. A standard European/English game where there are the normal English upper and lower case, comma, question mark, space and full stop can be harder here. You might have some luck here as I am told K, W and Y are mainly for loan words and don't appear in native words.
For the sake of clarity you will probably not be able to type in the accented characters normally on your keyboard and you will have to put whatever character you replaced instead.
For instance you want to write
Um dia eu desci a rua e peguei um sorvete.
but there was no i in the game and you instead replaced a # you found in the font with i it would run something like
Um d#a eu desc# a rua e pegue# um sorvete.
and the game would then display the proper sentence.
... probably should have chosen an example with one of said accented characters but hopefully you still get it.
The more advanced version will see you have to figure out how to add new characters to a game. On newer systems with fully realised font formats (I have an example for the DS in http://www.romhacking.net/forum/index.php/topic,14708.0.html
) this can be done more easily, for systems like the GBA which had not quite graduated to that yet then it was often still standard picture format approaches. This can be harder if you are not already familiar with hacking, and tends to be fairly tedious even when you are.