Browser: Chrome 64-bit, ver. 86.0.4240.111 on Windows 10
The strings “Subject” and “Body” in the new message form have not been localized - there no strings for them on crowdin or the strings are not used. While the Bulgarian translation is not 100% complete, I have also checked the Russian one which is 100% complete and the strings are not localized there either.
This string seems appropriate for “Subject”, perhaps it was even meant for this context (“root -> views -> messages -> subject”) but is not used:
There’re several strings for “Body”, this one could be used here: