In this case go to Extensions ➙ Language(s) ➙ Overrides, select the language used on the site then click on the New button.
Here you have to search for the value, which is "Choose!" in this case.
Then you have to copy the found language constant and insert it to the Language constant field, then just type your custom text into the next, Text field, and Save.
Overrided language constant in the dropdown container