There are little differences depending on the field of study:
Mechanical engineering: You can write one of two Thesis abroad. Either the project thesis or the bachelor thesis.
Industrial engineering: You can write your Bachelor thesis extern.
CES (Computational Engineering Science): Both scientific Thesis can be written (Project Thesis and the bachelor Thesis)
The Masterthesis can be written abroad for all fields of study including Simulation Science. It is important that there is a limit for the creditpoints you can collect abroad during your Master. For Mechanical engineering and CES this are 30 CP for industrial engineering there are 20 for the economic subjects and 20 for the mechanical subjects.
You need to find an intern supervisor. He decides about your grade, the extern supervisor can just make a proposal.
The projectthesis/ Bachelorthesis abroad can be written in German or English.
Often scientific thesis are in line with an internship.