I've always used attention to detail when writing (although I give a pass on grammar/spelling to some extent to non-native speakers) as a proxy for general professionalism/intelligence with relatively high accuracy.
If you can't be assed to write accurately and consistently in your native language (and/or don't avail yourself of the myriad automated tools to achieve this end), how can anyone expect you to be accurate and consistent in any other endeavor? It's not like language and written communication is unimportant; indeed it may be the most important skill any information worker can have.
If I were considering hiring a native speaker of English, a single homonym misuse (its/it's, effect/affect, et c.) or lack of subject-verb agreement in any pre-hire communication (emails, CV, their public-facing website, READMEs on Github) would be sufficient for a NO HIRE in my book.