He maybe starting to learn German and just doesn't have the time for both.I didn't say anything about "reaching proficiency" in a foreign language. I am very aware of "reaching proficiency" of another language.
I said...."I hadn't heard much improvement in his English for the last few months".
Maybe you should learn to read more carefully before replying.
As regards the 'interpreter', you will probably know that if someone beside you is fluent in the language that you need to understand, it is so easy to just rely on them instead of persevering on your own. It happens , as I know from personal experience.
De Zerbi is coach of an English Football Club, where the spoken English is vitally important when wanting your players to cope with the intricate style of football that he demands. I do wonder how many of his coaching assistants are Italian...?