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.