system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What makes non-native English speakers sound foreign when they have good grammar?

By "when they have good grammar", I mean that it's good enough that the grammar itself wouldn't be the tipoff that they're not a native, so what is?
Edit: I meant over a text-based medium, so also excluding audio tipoffs.
gpt
Native speakers of English tend to use a lot of words and phrases that are native to the region or culture that they learned English in. 

As an example, a native English speaker from Australia might use the phrase "they're right cunts" to describe a group of people that were being unruly, whereas an English-as-a-second-language speaker from Germany might call them "hooligans". It's these phrases and words that form the backbone of our language skills. Another easy example would be an American describing a punch from out of nowhere as a "sucker punch", but an Australian would call it a "king hit". 

It's the same language but used differently. Depending on where and when you learned the language, your choice of idioms and words will be slightly different, and it is incredibly easy to spot when compared to your own native culture.