Eurocontrol have approached the task for the European air traffic controller force in the correct manner. Their ELPAC examination is only designed to test up to level 5. This makes sense because a non native English speaker may suffer a drop in ability if they do not use the language on a regular basis. Eurocontrol spent over 3 years developing a series of examinations, trialing them over and over again and finally submitting the data to an independent university for verification. All that before they released the first examination version. Well done eurocontrol. Elpac = level 4 and 5. If an exam is only rated to level 5 a level 6 can not be issued. This is quite correct.
Interestingly though our ANSP decided to let us choose between ELPAC (only up to level 5) and RELTA (all levels available).