जानना is 'know' and क्या आप अंग्रेजी जानते हैं? wouldn't be wrong but for learnable skills such as languages, musical instruments etc, it is more common to phrase it as क्या आपको अँग्रेज़ी आती है (literally, 'Does English come to you?').
In addition, क्या आप अंग्रेजी जानते हैं? can also mean 'Do you know about English?'.
'Understand' would be समझना but admittedly it sounds a little weird to use समझना with a language. However, I suppose you can use समझना to ask if the other person understands what you are speaking in your language but is not necessarily able to speak in that language.