500. Changing language in Debian (GNOME 3, terminal) to e.g. fr_CH

This is both obvious, and tricky at the same time.

1. Install locales
The first step is to install the languages you need

sudo apt-get install locales
sudo dpkg-reconfigure locales

2. Optional: language packs
If you want e.g. libreoffice to use the new language, you'll need to install the corresponding language pack. Most programmes will NOT need this, but libreoffice and iceweasel do. The same goes for KDE.

sudo apt-get install libreoffice-l10n-fr iceweasel-l10n-fr icedove-l10n-fr

Basically, search for packages with l10n in their names.

3. GNOME
Select the new language in GNOME 3 by gong to Region and Language:

Now, here's the trick: even if the language you want is listed, click on the "+" sign in the bottom left corner, and select it like this:

While you now have two 'French' items, this is necessary or the language will not be correctly set. If you do NOT do this some things will change language, some won't (e.g. Activities won't change to Activités). In that case, if you log out, then back in and go to Languages you'll find that the language is set to "Unspecified [ANSI_X3.4-1968]". See the screenshot at the very  bottom of this post for an example of what it looks like.

Log out of GNOME and log back in again. Et voilà.

And...we have French! French everywhere.

4. Optional: set it in terminal
If you set the language in GNOME as shown above you shouldn't need to do this. However, if you are not using GNOME 3 you may want to set the language explicitly in your terminal by sticking the following in your ~/.profile or ~/.bashrc

export LANG="fr_CH.utf8"

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When it's NOT OK:

Note how it says 'Pays et Langue' but still says 'Activities' i.e. the language for some items has been changed, but not all.

See step 3 for an explanation of what's going on.