Thanks for raising it. However, I cannot reproduce the problem through on either my local site or https://sandbox.moodledemo.net/ . I've even tried to upgrade a site from Moodle 3.8 to 3.9 having en_us as the default language.
There may be some other error on your site that triggers this problem, please check your web server logs for errors, and enable debugging in moodle to see if you get any errors displayed.
It will also help if you could change the language to "en" or any other you have installed in your site and share which is the behaviour in that case.
Thanks in advance!