Hey, I will admit that I'm not super skilled in the details of translation... but this question really had me scratching my head. After reading the wordpress codex docs on translating, I was like "WOW these guys are real pros, this is great stuff..." (no I'm not really that surprised, just pleased), Anyway it should work. So it doesn't make sense that this wouldn't be translated.
Then I looked at the code in populate_options() again... and I think I know what the problem is.
I don't have time to research this completely and fix it, but Konstan, if you are already translating this then you may want to go in and offer a patch to the following (what I suspect) is a bug...
On line 231 of upgrade-scheme.php the code says...
add_option('blogdescription', __('Just another ' . $current_site->site_name . ' weblog'), __('Short tagline'));
Ok, so I think the reason it's not translating is that... __('Just another ' . $current_site->site_name . ' weblog') gets turned into 'Just another Foo-foo-blah-blah weblog'. Where Foo-foo-blah-blah is whatever your main site name is.
This then gets passed as the main lookup string for translation, which of course, you don't have in your .po or your .mo file.
So probably, the intent is for this to be implemented as a parameterized string where a replacement occurs.
I suspect that this bug has never been found because as Dr. Mike points out, it's really obscure since the eventual goal is that the user would replace this string with their own tag line. Although note that probably 40% of the blogs on the largest wordpressmu site (wordpress.com) still say "Just another Wordpress.com blog"... so we may be giving users a little too much credit.
Anyway, Konstan, good luck in working on your translation... and if you really want to fix this particular nuance, then I'm sure this is likely the problem.
Question for the Moderator: What is the correct way of handling a situation like this? Should I post a bug on trac?