What are the reasons for and against studying other religions? When I studied Spanish, I better understood my native English language. I wondered if the same kind of thing can happen in the realm of religion.

What are the pros and cons of studying other religions and their practices as a Buddhist?

To be clear, I do mean "studying" (and not "practising") another religion's practices... not that answers about the pros and cons of "practising" another practice wouldn't be most welcome, I just assume that practising another practice would be too confusing. Maybe someone will give an answer that challenges this assumption - Metta
In terms of general knowledge, it can be helpful to be familiar with the religions of the world on a basic level and there are many books which compare the world's religions in an easy format such as The Handy Religion Answer Book. Reading a book such as this gives an interesting glimpse into the vastness of the the world's religious traditions. 

Studying all or many of the world's religious traditions on an in depth level however, would take large amounts of time and effort which could take away from Buddhist practice. As people familiar with Buddhism, we can probably easily appreciate that even within a single religion there can be vast difference in practice and beliefs and to study multiple religions in depth would be a major undertaking.

Intention is everything. An intention to add a bit of general knowledge about a subject or to confirm that your current religion and practice is right for you may be wholesome. An intention to compare your own religion and practice against others in a conceited way or to avoid a tough part of your own practice by busying yourself with endless study could be not-so-wholesome. 

When preparing to leave the religious tradition I had been born into, I read a general book comparing religions and it was helpful for putting things into perspective. But, using the example of reading about meditation vs practicing meditation, one doesn't get a true sense of what a religion consists of by reading about it.