The higher the grade in something, the more better it is?
If you have ICAO Level 6 proficiency, you do not need to re-test at certain intervals as per the lower levels.
For the OP: Go to an accredited ICAO language tester for the assessment.
I did mine, since although I'm bi-lingual, my first (home) language at school is Afrikaans and second language was English.
Even though I got a distinction at school, it didn't count.
The test is supposed to be fair, and more conversation driven. You lead the examiner, and he/she will base their decision regarding your technical ability.
Also, it is aviation oriented, and not everyday or Shakespearean English, so prepare yourself for some circuit pattern-type scenarios.
If you get less than level 6, you just need to repeat the test every x years, so employment should not be an issue.