Night Rating Application
Hi Callum,
Have you completed the NR course to add it to an existing EASA licence?
If so, you should be using form SRG1126 v5. This has no requirement for any language proficiency, as you will already have that at level 6.
If you're applying for a NR at the same time as an EASA licence, then the language proficiency is certified on the course completion certificate your ATO will create. The language assessment can be done by your FE or RT examiner, assuming as you say, it's your native language and you're level 6.