Is it permissible for a girl to wear jeans with a loose shirt and a head cover that conceals the hair, knowing that I have no intention of imitating the Western style.
Note: I live in a Western country.

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

If this young lady is living with her husband in a place where none can see her, then it is permissible for her to wear anything she likes as long as she does not intend to imitate men or immoral disbelieving women.

However, if she is living with people other than her husband, she should wear clothes that achieve the required concealment. In our viewpoint, pants—in all their styles—unless they are worn under a concealing loose dress that totally covers them, do not achieve the concealment that is required from the Muslim woman because they do not conceal the size of the limbs as well as the body curves and charms. This actually leads to temptation and stimulation of desires. The woman, by doing this, jeopardizes her dignity and honor. 

Moreover, wearing pants without them being covered by a dress that conceals them is never the dress code of chaste, decent women. That is why it is difficult to perceive that wearing them is devoid of the intention to imitate disbelieving or immoral women.

Hence, if the chemise that is worn above the jeans pants is loose and long enough to cover the entire body, including the arms and the legs, then there is no harm in wearing pants underneath it.

However, if that is not the case, then it is impermissible to wear them in front of anyone other than the husband.

Perhaps—O Muslim girl—you realize that when Islam subjects the specifications of the dress of the Muslim woman to strict restrictions, it aims, thereby, to maintain her and preserve her dignity and honor. The matter is not as it is propagated by the enemies of Islam, who claim that the Islamic dress code is only meant for suppressing her freedom or interfering in her private affairs. Since you live in the west—as you have mentioned—it is a matter of surety that you have seen, heard, or read about the frequent incidents of violating women's honor that take place in these countries because they expose their bodies before everyone. Thus, they suffered, as a result of this, evil consequences from which wise people should draw a lesson.

Truthful is Allaah Almighty as He says (what means): {…then whoever follows My guidance will neither go astray [in the world] nor suffer [in the Hereafter]. And whoever turns away from My remembrance—indeed, he will have a depressed life, and We will gather him on the Day of Resurrection blind.} [Quran 20: 123-124]

We would like to draw the attention of this young lady to the fact that permanently living in these countries of disbelief is not permissible for the Muslim if there is no urgent necessity that might only be achieved through this. That is because of the dangers that face the Muslim there concerning his religion.

Allaah Knows best.