This is an old thread, but nothing seem to have changed. This is what I did:
1. Download the locale file to server.
2. Untar the file.
3. Move the language files to
4. Run the ./install
You will be asked for the squirrelmail location, which I answered with the above path. Then you will be asked to overwrite several english help files. I always answered no, but I think you can overwrite them since there does not seem to be a manual language setting for email users in squirrelmail.
I think you also have to configure your default language in squirrelmail. You can do this by going to /home/admispconfig/ispconfig/web/squirrelmail/config and run ./conf.pl . After changing the language default, you have to save and quit. I did this before installation of the locale, because I was hoping that my language was already present.
Now squirrelmail shows everything in my desired language.
Edit: I just noticed, that the thread author asked for squirrelmail in general. I think if your server locale is set correctly and then install squirrelmail, it will install the right locale for you. At least I cannot remember having installed locales for squirrelmail. As far as I remember it displayed the right language right away. But I have to admit that my etch server is still on English locale, so that this might be the reason for the need of locale installation.