I think this is a very personal decision that any Elements developer has to make. There are many factors that would affect your decision. I think the choice of a language should be made by the developer, but I have it easy since I’m the only developer and work for myself. I know this will be different in other situations.
First and foremost, are you a sole developer? Or do you have a team? Is the team familiar with Oxygene/pascal? Or are they more experienced in C#? Swift? Java?
One other thing to remember is that in Elements you can mix languages. One unit can be Oxygene, and a second could be a C# unit, and it will all compile.
Alan