Add description of language and number formats to pretty status.
This is shown in the summary page. Refactor a little to produce
the strings only in one place. Constify methods.
FIXES: #564
If the user selects a language from the dialog (by clicking 'change'),
then preserve that explicit choice even when clicking on another
location which would reset the language based on the installer-
language.
This allows for installing locale packages. This can be achieved
by adding a entry of the format packagename-${LOCALE} in the
packages configuration module.
