How to indicate the type of position of a letter writer?
Give your formal title. If the job description is roughly equivalent to Assistant Professor in the US (regular employee, not yet tenured) than say that, perhaps in a footnote: "The writer holds a position roughly equivalent to Assistant Professor in the US, but with no associated title).

People reading the application will probably have a rough idea of the difference between the German and US/Canadian systems. 

But, to protect yourself, verify the language with your head/chair.