Changing the language takes quite some time

[imobachgs]

Changing Agama's language can be a rather slow operation (about 20 seconds). The software service has a few things to do, but instead of displaying the progress on the overview page, it only displays the "Loading installation environment" message.

Eventually, it finishes and displays the overview page again, but it can be annoying (and the user might not know whether the process is progressing).

[imobachgs]

Now the user gets a progress screen so, at least, it is informed about what is going on (see #1375).