Support multiple languages (movie information)

[jsayol]

themoviedb.org offers all movie information and poster images in several languages. Add support to select the preferred language.

There should be a global option (linked to the app's selected language? #13) plus an option to set the preferred language in each collection, superseding the global one.

What happens when the user changes the global or collection-specific language setting? Some options:

1. Do nothing. All current information stays as it is, new information gets added with the newly selected language
2. Check the currently available information and fetch anything we don't have in the selected language
3. Ask the user what to do, either 1 or 2. In this case, if the user selects 1 there should be a way for them to do 2 later on if they choose to.