You can prove your proficiency in a language by one of the following means:
- language certificates with attainment showing level C
- university diplomas/transcripts obtained in the languages in question
- proof of bilingual education at secondary level.
Recommendation letters from professors/employers are not accepted as proof of the declared language knowledge. Mother tongue is also considered a 'very good knowledge', and no proof is required (nor points attributed, though).
All candidates must declare 1 mother tongue; no points are awarded for mother tongue and therefore no proof is required. You can declare up to 3 mother tongues. If you have more than 1 mother tongue and wish to receive points for them, you can declare these in "Other languages" as long as you can provide written documented proof.