Simplify language selector dialog

The design for the language selector is quite complex.

It shows languages in two formats - the entries at the top are language/country pairs showing both the currently selected/installed/common languages. Below these is an expandable list of all languages which are selected in a two step process (pick language followed by country).

This is both complex to implement and to use:

• The interaction between selecting languages from the top list and the bottom list is quite different.
• Expanding the list changes the behaviour quite differently.
• Should languages be in both the top and bottom list, e.g. if you have en_US selected should it be selectable in the "English" item in the bottom list?

I propose we simplify to show:

• One list of all language/country entries - one line per option.
• The "common" set sorted to the top in order of popularity. The remaining below in alphabetical order.
• A search field always shown that allows you to skip to a specific language/country combination.

If the search field is considered visually distracting then that could be hidden and an expansion row could expand to show all languages and the search field (i.e. only common set and installed shown by default).

Current design showing mixed behaviour:

Proposed design: