All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
One has to get ready for the the prayer in the best attire possible and in the most appropriate form, as Allah Says (what means): {O children of Adam, take your adornment at every masjid, and eat and drink, but be not excessive. Indeed, He likes not those who commit excess.} [Quran 7:31]
So, the more one is physically and spiritually ready for the prayer, the better his reward will be. As for the Western style of dress, such as the trousers and shirt, there is no blame on wearing them while performing the prayer on the condition that they completely cover the ‘Awrah (parts of the body that must be covered in Islam). Yet, if such clothes shape (outline) the two parts of ‘Awrah (back and front), though they cover them, then the prayer is valid but dislikeable. Another condition of wearing such kind of clothes is that they should not be transparent in a way that makes the part of the body they cover visible. If the part of the body covered by such clothes is dimly visible without showing the colour of the complexion, then the prayer is valid too. However, what we have previously stated is better to be followed.
For more benefit, please refer to Fatwa 209827, 81610, 92330, and 118902.
Allah knows best.