Should it not be: The girl and the boy speak Romanian? I.e. băiatul = "the" boy
"Vorbesc" is indeed the first person singular form, BUT it is also the third person plural form! So it means both "I speak" and "they speak"! Something I am noticing about Romanian verbs is that they often tend to have the same form for first-person singular and third-person plural. The version you've suggested would be the second-person plural form, I believe, i.e. "You all speak". So that would be grammatically wrong in this sentence.