Oh,nice question. Most people,specially teachers say that use just English dictionary. But I suggest other thing,let me to explain with an example. Your mother tongue is Farsi,when u hear or see a new Farsi word that u dont know its meaning, u look at Farsi dictionary.Why? Because u think in Farsi and know in Farsi too.It is in English or other languages too, if u can think and know in English so its better to look at English dictionary but if ur English level is not too high and u cant know and cant think in English, so it is better for u  to look at English-Farsi dictionary. But it is good to know that thinking in English is the highest level of English level that anybody can achieve it.Like as other languages.