Beast Boy's first language was not English. Nor was it Robin's, Starfire's, or Raven's. They all just learned quickly. Cyborg was the only one of the main five whose first language was English. Beast Boy's first language was the Swahili of the tribe from Africa he grew up in. He learned English from various foster homes and working with the Doom Patrol. BB still in fact has problems with his English today and sometimes if he is really tired will only talk in Swahili. The team makes jokes about his intelligence but they know it isn't his fault. All of his books are in Swahili except his English dictionary that he keeps close to his bed in case he doesn't know the right words.
Raven was born in a dimension called Azarath. She really isn't human like Starfire. Not only does she come from a different dimension, but she also is half demon so she comes from two languages (though she doesn't actually speak it, they communicate through their minds but in demonese). Azarathian was a language that sounded a bit like Latin., just backwards. She read books that were in the Azarath language and wrote all notes in it. She never really told anyone that in the back of the Book of Azar she had photo copies of an English dictionary to check the words she used sometimes. However she never slipped up like Beast Boy or Robin did.
Robin was born in a caravan while the Haly Circus was on the road in Europe. Everyone in his troupe knew English, but as the usual with Gypsies they only talked in the Romani language while together and only used English when performing for people. Now Robin knew enough English so that when his parents died he could talk to Bruce and Commissioner Gordon and explain what he saw, though at the time he didn't understand Bruce's obsession with him. When he left with Bruce and stayed at his house, it was quickly found out that he didn't know English as well as they had hoped, so he was taught the basics before he was shipped to school; but even with all of this, he still slipped into Romani when training or heading to the circus to talk to the acrobats and the owner to see if he could take a swing on the trapeze. They usually said yes when they heard the name "Grayson" even now he was the best trapeze artist the world had ever seen. Now Robin, back at the tower, still didn't have a great grasp on writing in English so everything he wrote was in Romani, like when Starfire wrote in Tamaranian.
Now everyone knew that Star didn't know English at all almost but they didn't know that as she taught Cyborg Tamaranian, he was teaching her English to which the three other Titans would join in saying they were bored and there was nothing to do in the tower, but Starfire and Cyborg saw them frantically write notes and leave mouthing the new words under their breath. The Titans came from all over the universe and from all different cultures and languages. They each have their own language to speak that reminds them of home and the Titans love it.