Something I have felt like bringing up for a while is what I've been referring to in my head as "hidden accents." I think that you can subconsciously assume what someone's first language is from their pattern of speech in English, even if it is perfectly grammatically correct/makes sense. Obviously, it's not pinpoint accurate, but there are some trends.

French speakers... love the word "were." It's "le subjonctif," technically it exists in English but native English speakers don't use it nearly as much as native French. This comes along with French speakers also abusing "It is." "It is important that..." "It is good that...", I swear it's everywhere when you start paying attention. It is not that it is not normal to use "it is," but that it is thrown into everything.

Many Germans just add random spaces in-between compounded words. I will call this the "Antibabypillen" effect, because it's the first silly German word than came to mind. These spaces occasionally take the fancy form of dashes (-). Also, their grammar is sometimes far too perfect, it's offputting.

Arabic speakers... Yes, okay Sacha, we know blah blah blah, but the main thing with Arabic speakers is creating ungodly Godzilla of sentences that overuse things like "and," "but," "or," or all of the above... lol, this is me. This happens with others as well, but sometimes our word choice in English is awkwardly formal/messes with connotations. I can't think of a good example right now, but I'm sure you can stumble across one easily.

It's not just grammatical errors or poor word choice, but also structure/flow. Not sure if anyone is interested, but I have always wondered if other people notice this too, and what they have noticed, considering many people here are not native English speakers