i18n: Make sure to set locale according to environment

This does not always seem necessary but is generally safer, see https://www.gnu.org/software/libc/manual/html_node/Setting-the-Locale.html

For example, I had to specify this for Mousepad when I switched to GtkApplication. If Mousepad is started with a specified locale on the command line, and a translation takes place between the creation of the GtkApplication and the emission of the "startup" signal (typically in init()), then without this change this translation does not take into account the specified locale.