If you want to add your language to an already running (English-only) browser, you must know, which release you have:
"Help" and then "About Seamonkey" or "About Firefox" will show you the release.
Open this link in a new window and choose your release (see above).
On the page "Index of /pub/mozilla.org/seamonkey/releases/x.x.x" choose:
Open this link in a new window and choose your release (see above).
On the page "Index of /pub/mozilla.org/firefox/releases/x.x.x" choose:
linux-i686/ (or linux-x86_64/) and then choose ex: for German:
While downloading you will be asked (in English of course):
Install Software...
Install now
Restart now
If the browser continues running in English:
Edit > Preferences > Appearance > Language > German
then close browser an relauch it.
Additional "large" applications like firefox should be installed at /mnt/home/opt/.
Information about other extensions can be found here.