Modify default translation/texts (en) via theme


I want to modify the text-hint, which is shown, when a user clicks “password reset” at log on:

A password reset message has been sent to the e-mail address of this account. If you do not receive it, check your spam/junk folders or ask your local administrator for help.

The modification for german language was done by editing the files de_DE.js and de_DE.json. The modified files are located in /themes/custom/core/l10n. de_DE is set as standard. My custom theme is activated.

Now I would like to modify also the default en-text, but there is no en.js/en.json file. I have also tried to understand the Factory.php which seems to be responsible for the translation management, but based on this I would maybe expect an en_US file (which is also not there).

Any ideas on this?