Internationalization (i18n)

[billz]

This adds support for internationalization (i18n) and localization (l12n) by using GNU's Gettext to manage multi-lingual messages in a standardized way. It's assumed that the user has configured a language package on the RPi corresponding to one of our supported translations.

To list languages currently installed on the system, use $ locale -a at the shell prompt. This should return a list like the one below:

C
C.UTF-8
en_GB.utf8
en_US.utf8
fr_FR.utf8
POSIX

To generate new locales, run $ sudo dpkg-reconfigure locales and select any other desired locales. Important: UTF-8 is the preferred encoding.

The new file includes/locale.php sets the user's current locale using a very simple method. At the moment French (fr_FR) is the only supported translation (it could almost certainly be improved 😉). The default is en_US. Alternatively, it could be set to en_GB as this locale is enabled by default on Raspbian. So far volunteers have offered to translate German, Italian and Spanish #121.

The advantages of making RaspAP a multi-lingual project are obvious. The downside is the extra burden of maintenance going forward. Any future addition or change to a label in the UI will require localization into each of the supported languages. The good news is, this can be done by editing the Portable Object (.po) files centrally and recompiling the Machine Object (.mo) files.

Thoughts?

Resolves #122