Replace useless "Learn more about add-ons" in Languages pane

[nohamelin]

Addons Manager, Languages pane, with a empty list of language packs:

The button is a link to the "Get Add-ons" pane, where no info about languages packs can be found. Useless.

The plan is:

• Rename it to something as "Learn more about language packs".

• Link it to a more complete, localized, web article. Sadly, I can't found anything "official". The closest thing is: http://kb.mozillazine.org/Language_packs (only Eng)

  I'm thinking to writing a new article for SUMO about it. It can be reviewed and translated with the help of the community. But it will take time.

[nohamelin]

It still needs work, but it's a good start: Install language packs to get more languages for the interface of Firefox | SUMO

[nohamelin]

Done: 35e3dd7