Why isn't 'Hva språk snakker du' OK? If not, wouldn't it be better to have the English say 'Which language...'?
'what' is more commonly used than 'which' for this expression in English.
"Hva språk snakker du?" would not be accepted in Bokmål, but might be found in certain dialects. This should just show that you may use different question words for certain expressions as this one.
Is språk unchangeable in plural?