That said, if you have a foreign accent people expect a certain (low to average) level of fluency and can get quite confused if you use native idioms or slangs,

That said, if you have a foreign accent people expect a certain (low to average) level of fluency and can get quite confused if you use native idioms or slangs, which they only expect from native speakers with a native accent. There’s no way to pull off an “innit” unless you speak like a native Brit. Vocab is easy to learn, losing one’s native accent is exponentially harder the older you learn a language, and as you probably know only through hard work with an accent coach you can eliminate that uncanny valley between fluent second-language speaker and native. Also, not everyone wants to invest in a coach unless they really have to. You can learn a language on your own, not everybody has time, money and need to hire an accent coach.