Currently, almost all of the validation messages follow the same format, which I personally like.
Some of the examples are:

This value should be of type {{ type }}.
This value is too long. It should have {{ limit }} characters or less.
This collection should contain only unique elements.
This is not a valid International Bank Account Number (IBAN).

Basically, the format is: reference the subject ("this value", "this collection", "the file"...) and then explain what the actual problem is.

There are two exceptions that I'm seeing:

1. addressing the client only in some places. In the majority of messages, they have been constructed in such a way that no single individual is being addressed, whereas some of them do.
   This PR fixes this in a way so that the messages which have been addressing anyone no longer do. Examples:
   • "The value you selected is not a valid choice." is now "The selected value is not a valid choice."
   • "You must select at most {{ limit }} choices." is now "At most {{ limit }} choices must be selected."
   • "This password has been leaked in a data breach, it must not be used. Please use another password." is now "This password has been leaked in a data breach. A different password must be used."
2. avoiding being consistent with aforementioned format.
   This PR fixes that in a way that so that the messages conform to the same format as most of the others (referencing the subject and then stating the problem). Examples:
   • "Invalid card number." is now "This is an invalid card number."
   • "Unsupported card type or invalid card number." is now "This is an unsupported card type or invalid card number."

Also, as an added bonus, since I'm Croatian, I've done the translations for Croatian language in the very same PR + fixed some of the messages that didn't account for Croatian pluralization rules (or have, but implemented incorrectly).

All comments and feedback are appreciated.

Have a nice day!