[apparent]

Well, all English belongs to a dialect (British, American, subgroups thereof), but I wouldn't say that all English is accented. I'm usually OK with British and Australian, but would need to slow it down a bit for Irish or Scottish. I definitely need to slow it down for people with accented English, depending on how noticeable their accent is.

[schrijver]

Right, but if you were Irish, you wouldn’t have to slow it down when listening to English in an Irish accent. Your distinction to what is accented or not seems personal to you.

Accent is a subset of dialect, referring to pronunciation, where dialect also refers to vocabulary and grammar. It follows that if all English belongs to a dialect, all English has an accent.

Of course, some accents are more readily understood across the world; I imagine you are from the US, and there is the American English featured in film and TV (that I imagine synthesises different regional accents), that has a huge cultural reach, so people who learned English through media might find this particularly easy to understand.

However, outside the US, if you speak like this, you will always turn be regarded as the person with an accent, because you don’t speak like the locals. And people are going to find their local accent easier to understand.

[apparent]

I'm not disagreeing with your suggestion that a mechanism would be needed to set the "preferred accent/dialect" for a listener. My point is that when I was referring to accented English, I was referring to people for whom English is not their native language, for example a native French speaker. I know that people speak English in different ways in different countries, and that there isn't one "correct" English. But I was referring to people who speak English with a foreign-language accent in particular, who are the most difficult for me to understand at high speed.