How to translate form.php language message ?

I am writing a multi-laguage web site.

Q1 :
The form.php file contains:

$message = array(
	'success'			=> 'We have **successfully** received your Message and will get Back to you as soon as possible.',
	'error'				=> 'Email **could not** be sent due to some Unexpected Error. Please Try Again later.',
	'error_bot'			=> 'Bot Detected! Form could not be processed! Please Try Again!',
	'error_unexpected'	=> 'An **unexpected error** occured. Please Try Again later.',
	'recaptcha_invalid'	=> 'Captcha not Validated! Please Try Again!',
	'recaptcha_error'	=> 'Captcha not Submitted! Please Try Again.'
);

Is there a way to dynamically replace these values ?
I'm new to PHP, but in C++ we vould pass an argument to the constructor of the class or add a setter. Is it possible to do something similar ?

Q2:
I have 2 dates fields in the form, where is the validation code of the form ? I need to add code to meke sure 1st date is inferior to 2nd date.

In addition, the recipient do need to receive the mail with french date format dd/mm/yyyy, whatever the dispayed format on the form.

Thanks a lot.

Thanks a lot, i'll give it a go :)
About yhe daterangepicker, I'm afraid I read somewhere it does not support translation, at least for the time being.

I used a workaround for the date range: date picker for the start date and slider for the number of days...

Thanks for the trick for the message translation, it works fine :