How to indicate the type of position of a letter writer? I am faculty member of a European university and I was asked by a student to write a letter of recommendation for graduate admission to a programme in the US or Canada. 

Should this letter contain an information about the type of position the letter writer is holding? In my case this is a tenure track position but this fact is not stated on the department website. Should this be included in the letter? What would be the ideal way of including this? 

To make matters more complicated, the type of position I have is "Assistant Professor" but the regulations explicitly forbid to use this as a title of the person holding the position.
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.