An interdisciplinary degree is one where you can create your own degree by choosing modules from different departments across the university. I know that the University of Stirling has a very flexible approach which is similar to this. Alongside your main degree subjects you can choose practically any undergraduate module from any department at the university in your first two years, as long as you have the academic qualifications needed for the modules in question. So, for example, you could do two languages and sociology or sports studies or biology or psychology. The problem with Stirling is that it doesn't offer German, only French and Spanish.
A single honours degree is where you spend 100% of your time studying one subject. So, for example, if you did a single honours degree in French, all of your modules would be connected to French in some way - French language, history, culture, literature, etc.
A joint honours degree is where you spend 50% of your time on each of two subjects.
And an honours degree with a major is where you spend 75% of your time on your main (major) subject and 25% of your time on a secondary (minor) subject. (Some universities split this 60:40.)
Lots of universities give you the flexibility, especially in your first year, to take a subject that is completely unrelated to your degree course.
Applied languages courses focus on learning the language and on other vocational uses of the language, so there would be very little, if any, focus on literature, and you would usually have a choice of modules. Here is the link to the programme content of Heriot Watt's French and Applied Language Studies Course, to give you an idea:
http://www.undergraduate.hw.ac.uk/programmes/R100/In this course your main language would be French but you can also continue studying German as a secondary language (the other way round is also offered) and then choose from a range of other modules. Economics is one of those but you don't have to choose that if it's something you're not interested in.