"My sister finally learned English."
Translation:Mia fratino finfine lernis la anglan.
I don't know if I understand you perfectly.
angla means English, as in something relating to English people, see anglo = English person.
la angla is short for la angla lingvo, and therefore whenever you are talking about the language, as we are in this sentence, one must use the definite article to differ between the language, and the adjective describing something related to the English people.
Mia fratino finfine lernis anglan. would not be grammatical as it would lack a noun, just like saying: my sister finally learned how to make sweet
Though when you write la angla you are essentially bypassing this as it as previously mentioned is only a shortened form of la angla lingvo.
Hope this helps.
Use of the article in Esperanto and in Spanish does not come naturally to me. My problem comes from in that we don't say "My sister finally learned the English," but we could say that "The English always ..." meaning the English people. We can also say things like "The book was invented ..." meaning that books were invented. Also what bothers me is the use of the article in front of possessive pronouns, as in "La dua libro estas la mia".
The main difference between the use of the definite article in Esperanto and in English is that in Esperanto the article, with a singular noun, may be used to indicate an entire class.
I'm not sure I understand exactly what is meant by entire class, and if it relates to this issue in particular.