Yep.  And when it's the other way around (good English suddenly turning into poorly-written gibberish) it's also a bright red flag to members who've been here a while and are well aware that bitcointalk accounts can be quite valuable and that they change hands more often than they should.

Honestly, from my experience it's extremely rare for new members to be really proficient in English, and I think that's been true since I registered in 2015.  And we all know why: sig campaigns and bounties, which pay for posts in English, draw members who need to earn money in droves--and I'd venture to guess that most are from Asian or African countries where English isn't most people's first language. 

Note: I'm not knocking anyone for wanting to participate in campaigns or even for writing in a language that's not their native one.  The problem I've always had is that poor writing is often combined with a glaring lack of thought or effort.  You'd think that if you want to maximize your earnings long-term you'd do your best when posting, but Jesus....just look at some of the posts in Altcoin Discussion or other spam magnet sections.  So many of the lousy English writers aren't even trying.