There are many churches in Barcelona so if you are staying in the city, I´m sure you will find one just by walking 5 minutes. I remember spening quite some time at two different churches as well as the church called La sagrada familia. Both of these two regular churches had mass every night during our stay (as far as I know). If you know some Spanish, I would consider to check whether on the mass are in Spanish or Catalan, because that is various between the different churches.
A lot of people in Barcelona aren´t really with speaking English, so skills in Spanish will help you. Some people speak good, but some doesn´t speak it at all.
I hope you will enjoy your stay!