Translation of Standhaftigkeit

I try to learn English as much as possible so that I can travel next year to the United States. I would agree to your proposal but I also need to find good teachers. My goal is quality and speed.
The fact that James is active in many industries in Colorado, does not mean that the same must apply abroad, let's say in Spain. This fact complicates the ability to learn English in everyday life.
I have learned German for one year and I would like to intensify my studies. Do not know how yet... It is very difficult at my age to start new projects in life. I am over 75, did I tell ya...