There is a difference "Do you speak Bangla?"= "আপনি কি বাংলা বলতে পারেন / আপনি কি বাংলায় কথা বলতে পারেন?" "Do you know how to speak Bangla?"="আপনি কি বাংলায় কথা বলতে পারেন?" ***"Know"=জানেন; However, saying -'আপনি কি বাংলায় কথা বলতে জানেন ?" is rude. "পারেন"=indicates "ability", "can you speak?"

apni ki bangla bolen? ( do you speak Bengali?) *apni ki bangla bolte paren? (can you speak Bengali?) *apni ki bangla bolte janen? (do you know how to speak Bengali?) *these two can be used as insults if you stress wrongly. If you speak with stress on the last word, it would become a rhetorical question and an insult. Use no stress. e.g. apni ki bangla bolte JANEN? (do you KNOW how to speak Bengali? or I don't think you know how to speak Bengali.) apni ki bangla BOLTE janen? (do you know how to SPEAK Bengali? or You speak vulgar Bengali. or You speak Bengali poorly.) Same with PAREN (CAN), it will be mildly insulting or joking even.