where can I edit the default text of the reject letter ?

[TheDragon]

see subject
thanks

[Kindred]

language files... not sure which one though

[Illori]

i would guess email.<language>.php as almost all emails are in that file.

[TheDragon]

^^ close enough to find it
THANKS

EmailTemplates.english.php

[Burke ♞ Knight]

see subject
thanks

That is not really a good way to ask for support.
Being a little more descriptive of what exactly you'd like to do, would be a lot better.

[Steve]

Usually that's true BK but in this case the title really does say it all.

[Burke ♞ Knight]

Be that as it may, being descriptive in both title and post, are better ways to get the answers to your questions.