Hello, I'm the author of that post Dr.DragonEye mentioned.

Currently the easiest way to get Latvian keyboard layout on N900, is to simply install ukeyboard and select Latvian layout from Settings -> Text input -> Hardware keyboard layout. Then you can type Latvian letters by pressing "," fallowed by any letter.

I'll quickly explain how to use those configurations you see on my blog post anyway. You need to modify the "rx-51" file, located in "/usr/share/X11/xkb/symbols/nokia_vndr/" directory on your N900. Just append the layout configuration to the end of the file, and then activate it by typing "setxkbmap lv" in the terminal. You can go back to the original layout by typing "setxkbmap us". You will need to run setxkbmap after every reboot, I'm not sure where you can change the default layout on N900. Just be careful, don't reboot if setxkbmap isn't working after your modifications, N900 probably will not boot with messed up "rx-51".

It should be easy to modify my last configuration from Latvian to any other language, that consists of Latin letters and not more than one other letter per each Latin letter.