Language of applications are not translated if the default language is changed

[john doe <johndoe65534@mail.com>]

Hi,

I have install Debian 9 using as the default language 'C'.
I want to add some new languages, and for this I do 'dpkg-reconfigure 
locales'.
I'm currently using Gnome and Mate.

How can I add support for those new languages so all applications will 
be translated in the desired language (using command line is prefered)?

--
John Doe