Hi Michaël,
I will post a snippet later. There is a hook that allows you to do it.

bpmts_form_field_validation_message

Regards
Brajesh

Hi Michaël,

Here is an example.

/**
 * Custom validation for BuddyPress moderation tools.
 *
 * @param string $err error message if any.
 * @param string $field field name.
 * @param mixed $value field value.
 *
 * @return string
 */
function buddydev_custom_moderation_validation( $err, $field, $value ) {

	// $field could be 'subject', 'message', 'category', 'name', 'email'

	switch ( $field ) {

		case 'message':
			// validate and if error, set to $err.
			if ( strlen( $value ) > 500 ) {
				$err = "Only 500 charaters allowed.";
			}
			break;
		case 'subject':
			// if subject is longer than 50 character.
			if ( strlen( $value ) > 50 ) {
				$err = "Subject is too long. Only 50 letters allowed.";
			}
			break;

	}

	return $err;
}

add_filter( 'bpmts_form_field_validation_message', 'buddydev_custom_moderation_validation', 10, 3 );

Please change the values as you please.

Regards
Brajesh