Agreed. The best example of this for me is nextdoor. The spelling and grammar for most of the posts are terrible and the posters are predominantly white and over 50.

The kind of mistakes a non-native English speaker makes tend to be different than ones an uneducated native speaker does.

I think that lack of an article 'the' is typical of native speakers of Slavic languages?

Slavs aren't the only ones though! Romance languages, while not completely lacking articles like Slavic ones, have slightly different requirements for articles. (They tend to use them more for abstract concepts, eg. "the reality" when we would say "reality", or "the <noun>(s)" for describing general behavior/attributes of that noun -- this difference may make speakers over-correct by using articles less in English)

Certain nouns are "inherently definite" -- names of people or places, for example, or abstractions or things which are inherently unique. Languages which mark definiteness often differ as to whether these inherently definite nouns, or which ones of them, should be marked as such. English generally treats them like proper names -- no article, definite or indefinite. Romance languages more often require the definite article. These are just two different ways of indicating the same underlying category.

It is. It took my father ~20 years of working in an English-speaking country to be consistent with his usage of that article.

If you're going to involve someone specifically for the purpose of writing English, then obviously you'd pick someone who's good at it. It's not a native/non-native distinction so much as it is "good at writing in English" vs not.