好急......grammar

一定係 is
其實你的is/are (verb to be) 係講緊你的proficiency ，所以用is 先o岩既~
同埋因為proficency 係uncountable ，所以係is

because "my proficiency" is single and "in both languages" is perosition phrase, thus it is use "is" and no "are".

應該係 are , 可能因為係被動式 word 用左 my proficiency 做 subject.

應該用 is
因為你係prove 你既proficiency in language
而唔係 prove language
所以 proficiency 先至係subject

而proficiency 係singular
所以應該用 is.

Therefore, my proficiency in both languages (IS) proved

"IS" uses for the "my proficiency" not for both languages...

Right !

因為個subject是proficiency（單數），不是both languages ，所以應該用is。