What should be included in a motivation letter while applying for an MA (in IR or development studies) course in Europe, keeping in mind that I am an engineering student in India with a minor degree in humanities (PPE)?  

A Letter of Motivation is a letter which accompanies your application and acts as a means of introducing yourself to the programme coordinator and the admissions committee. 

Your letter should contain specific information about why you think that you should be given a place in the programme.  You should detail your skills and explain the reasons why you are a good fit for the programme. Also, your letter should serve to convince the reader that you are sufficiently interested in the programme to merit further attention and a very careful screening of your complete application package.