Let me tell you a story.
For 9 years ago I couldn't speak nor understand English. I killed boars and spiders to level up. Of course I did those easy quests like "kill 5 spiders". But I couldn't do even the most basic ones like I can now. For example "turn in the harvest stone to barker in Tanaris". I couldn't read, so I was lost.
I only got to level 54 in vanilla by doing this, and let me tell you, it was really hard back then.
So how did I learn English? Well. I kept playing. I played a lot actaully. I had fun playing as well, which was the main reason why I kept getting better at it. I'm by far not the best English speaking player around. But that's actaully a good thing. Why? Cause it's my motivation to get better.
So my advice for you: Change the language to English and play a lot. Interact with the English community more. It's the best way.
You can also go the extra mile by reading books, watch movies, pay for English lessons online etc.
My question to you though is: Do you have fun by actaully reading books/watching movies? Cause having fun is the only thing that keeps me going on to get better (at least for me).
Hence why I barely learned anything from my English class lol.
Hope this helps! :)