HELP PLZ, I need to edit subtitle, thats all

PLEASE, HELP ME!
(my english is kinda poor)
I have some matroska(mkv) and ogg(ogm) files
They have two audio, japanese and english, and english subtitle
I just want to edit the subtitle, adding my language to the list(not erasing the english one)
But I have no idea how to do that...
Do I have to split it into AVI etc?
How do I do this?
I really don’t know how to do this, please, help me here... I do speak english, but my friends don’t, and I want to translate it to portuguese, but I dunno if I have to convert the image and then encode it again, and I don’t know the softwares to use

Thanks in advance
Jonathan

To edit mkv use virtualdubmod
http://virtualdubmod.sourceforge.net

To download virtualdubmod you can do it directly here
http://sourceforge.net/project/showfiles.php?group_id=65889
if you look at the bottom of the page you’ll see a language pack. download it too to have virtualdubmod work in your language

http://www.matroska.org/index.html.pt
This has matroska help in portuguese

Can you say what to do with it?
(I didn’t download it yet, I am at work)
Because if I use this program to extract it, I’ll have to encode it all again into one mkv?
or I can just edit the subtitles?