Advanced Language Testing

Based on the concept of certified linguistics acquisition testing.

Then, the components are likely: retension (memory / range), liquid intelligence (forming variety of sentences), and formal concepts (grasping deeper parts of a specific language).
This would be tested for more than one language.

Questions to simplify are asking What languages you speak, and then assessing how challenging those languages tend to be.

In terms of English language, here is a ranking of how challenging languages are:

Least -->
Basic English.
Intermediate English.
Advanced Spanish Basics
Latin roots (some prefixes and suffixes)
Greek Alphabet
Very Basic French.
Some grasp of Polish humor
Some Latin
Intuitive Language
More Advanced French
Some German roots
Fake Italian
Basic Chinese (big step)
Some Hieroglyphs
Rosetta Stone type thinking
Beginner Reading Arabic
Intuitive Japanese
Essence of Korean
Universal Languages
Basics of Reading Ancient Mongolian.
Very Basic Italian
Some Turkish grammar
Super-Advanced English
Intuitive Hebrew

Ancient Hindi

Golden Secrets
Grasping Romanian
Russian
Integrated in French culture
Integrated in Chinese culture
< -- Most