What recommendation can you give for the image sizes in pixels if I want to look at the images in full screen? e.g. height at least 1500Px.
Well, there's nothing complicated about that. You want to use original images and scaled images that are large enough to fill a typical monitor. Hitting 1920x1080 would take care of at least 95% of all your site visitors. Bumping it up to 2560x1440 would include iMac 27 users in the "good show" department, though you'll find that the scaled images start getting pretty large (file size).
Is it possible to offer a language selection within the skin so that the visitor can select his desired language?
The skin doesn't have that capability. In fact, I think there's only one skin that does-
Polyglot. It's an extremely messy thing to try to do, largely because the jAlbum core doesn't have a built-in way to handle two sets of tooltips, two sets of button labels, two sets of titles, two sets of comments, and so on. So, the skin has to come up with some sort of kludgy method of doing it.
About the only thing you can do is make the album - in English, for example - then make a copy of the project, and change everything - to French, for example. On your site, you'd have to provide a way for a visitor to choose the version he wants to see.