Regarding this question I understand that the way of translating a theme is:
The question is, it is possible to change the default language and the variables translations from the panel?
That might be difficult, see also this thread: Translation and config.php Bug?.
Edit: Also, I haven’t actually tested how Kirby behaves if you have only one language but want to make another language the default language. What happens to the text file extensions? I don’t think this is a really stable thing to do.
Yes, I agree, the text file extensions would be a problem.
The idea is define the default language just once before start generating content. Then I think is better use the config file.