Problem appears only if typing non-ascii from keyboard, to any field in message composer window - body text, subject, To/Cc/etc fields.
As for me, I experience it for russian text and 3rd level characters, such as euro and per-mile, while english entry and more standard punctuation are ok.
However, problem doesn't appear for copy-pasted text.
I'm subscribed to claws-mail users mailing list, and it seems also sending bug-related messages (begining from [Users][Bug N]). Can't find anything about this problem.
Please try running claws-mail --debug from the console and see if it output any error when you are typing into composer window.
Problem disappeared and doesn't appear also without any options, as well as starting by desktop launcher.
TLDW, there is full answer (along with all question thread in CM users mailing list):
For summary, problem seem appeared along with untranslated folder names like Inbox, Sent, Trash. I can reproduce it by running with LANG=C, but unsure, wether it was real reason. The most I remember, it appeared after upgrade from 3.17.3 to 3.17.4.
Finally I found at least one bug trigger for this version. I use LiteHtml to watch html messages (usually in rss/atom news). Its functionality seems just enough - though not as exact at rendering as dedicated www-browser, at least it has not problem, when some text parts disappear or are scrambled (as happens in text mode).
Bugs happens when just switching to html message view. Its effect seems to be locale resetting to some default (probably C). One switch is enough. Still have no way to undo bug without CM restart.
About symptoms. It seems, that folder names don't update to unlocalized instantly. Still did not trace, when it happens - I just know, it happens over time. The only places, where it is visible, is new message composer (open new empty with Ctrl+N, so that default window name appears, which should be localized) and desktop notifications, e.g. about new incoming.
I wrote to mailing list as well, though still no reply - probably they lost interest after being unable to reproduce it (still not at their bugtracker).
I just don't know after what upgrade, because noticed it just now. Now I can switch to LiteHtml view and message composer still opens with my language, accepting all charset.