Handle missing "Language & Region" plug

[electricduck]

The Language & Region plug is not included with Sodalite — or any non-Ubuntu distro for that matter — due to it's huge dependency on apt (which it will essentially not work without). This is something we've worked around with the introduction of the Locale Selector app, however, a problem occurs when trying to select the language from the User Accounts plugin (as mentioned in #76): nothing happens and a warning is thrown in the console:

(io.elementary.switchboard:4931): switchboard-WARNING **: 19:32:32.139: Application.vala:98: Specified link 'language' does not exist, going back to the main panel

This is rather confusing for users and we should try and open the Locale Switcher if the Switchboard tries to direct to a panel named language.