Steps to reproduce the issue

Set site language to your language (in my example nl-NL)
Edit mailtemplate for your language
Save mailtemplate

Expected result

Use of language constants as defined in mailtemplate

Actual result

System information (as much as possible)

Joomla 5.1.4
PHP 8.2.12

Additional comments

This is caused by the fact that mailtemplate for language is stored in database

When removing entry from database, result is ok again.
This issue was reported already several times
#43503
#42353
#39228