Is it permissible for her to study in a mixed environment in a non-Muslim country?

Firstly: 

First of all, we appreciate your keenness to make sure that your actions are sound and in accordance with Islamic teaching. This is what every Muslim should do, and it should be done before starting to do what he wants to do, whether it is business, travel or a job, because true sincerity in following Islam means asking about the ruling on an action before embarking on it. 

Secondly: 

The scholars have issued fatwas stating that mixing in places of education and elsewhere is prohibited because of what it leads to of negative consequences and things that are contrary to Islamic teaching. The scholars of the Standing Committee were asked: 

Is it permissible for sisters to enter and learn in mixed schools and universities, because in the West there are no options except mixed classes, but the sisters wear Islamic dress and, moreover, the disbelievers cause them annoyance. 

They replied: 

Free mixing of men and women in educational settings is haraam and is a great evil because of what it involves of temptation, spread of corruption, and transgression of sacred limits. What happens because of this mixing of evil and moral corruption is among the clearest evidence for it being prohibited. If we add to that the fact that it is in a non-Muslim country, then it is even more emphatically prohibited. For a woman to learn in schools and universities is not one of the cases of necessity because of which that which is prohibited may be permitted. She has to learn in ways that are safe and free of temptations. We advise her to make use of sound audiotapes that are issued by scholars of Ahl as-Sunnah, and we also advise her and others to listen to Noor ‘ala ad- Darb, which is broadcast by the Idhaa‘at al-Qur’an al-Kareem radio station in the Kingdom of Saudi Arabia. 

Shaykh ‘Abd al-‘Azeez ibn Baz, Shaykh ‘Abd al-‘Azeez Aal ash-Shaykh, Shaykh Saalih al-Fawzaan, Shaykh Bakr Abu Zayd. 

Fataawa al-Lajnah ad-Daa’imah (12/181-182) 

And they said: 

It is not permissible for a Muslim student to study in classrooms in which there is mixing between men and women, because of what that involves of great temptation. You have to find a way to study in a place where there is no mixing, so as to protect your religious commitment and your honour. “And whosoever fears Allah and keeps his duty to Him, He will make a way for him to get out (from every difficulty)” [al-Talaaq 65:2-3]. 

Shaykh ‘Abd al-‘Azeez ibn Baz, Shaykh ‘Abd al-‘Azeez Aal ash-Shaykh, Shaykh Saalih al-Fawzaan, Shaykh Bakr Abu Zayd. 

Fataawa al-Lajnah ad-Daa’imah (12/173).                                  

Even if your husband agrees with you studying, this does not make it permissible for you to study in that mixed environment. 

The scholars of the Committee said: 

Mixing between men and women in schools and elsewhere is a great evil and serious problem that affects both religious and worldly interests. Therefore it is not permissible for a woman to study or work in a place where there is mixing between men and women, and it is not permissible for her guardian to allow her to do that. 

Shaykh ‘Abd al-‘Azeez ibn Baz, Shaykh ‘Abd al-‘Azeez Aal ash-Shaykh, Shaykh Saalih al-Fawzaan, Shaykh Bakr Abu Zayd. 

Fataawa al-Lajnah ad-Daa’imah (12/156). 

These are the fatwas of the scholars that you wanted to know. You can learn English from a female teacher who comes to your house, or from audio and video tapes and books, which are readily available in the markets. You can also join a university or institute and take the exams by correspondence or in the presence of your husband, and thus attain your goal of learning the language and obtaining a certificate, without going against Islamic teachings. 

We ask Allah, may He be exalted, to enable you to do that which pleases Him. 

And Allah knows best.