Configure the display language of the GUI

[cmihai]

Is your feature request related to a problem? Please describe
The Bitcoin Qt wallet supports more than 100 different locales. While we expect that most of our users know English, a polished application will allow them to select the display language most familiar to them. See also issue dtr-org/unit-e-project#44.

Describe the solution you'd like
The wallet settings page presents a drop-down list, allowing the user to choose another display language. Ideally, the application does not have to be restarted to update the display.

Additional context
The official Angular documentation has an excellent guide to internationalization.