Since #30931 many new messages start with "Please enter…", like "Please enter a valid email address".

I suggest to avoid these kind of messages, and to replace them with messages like "This value is not a valid email address".

Reasons:

1. At the moment there is a mix of both kind of messages. In the same form you could get the message "This value should not be blank." on one field, and on another "Please enter a valid email address.". I would prefer to get consistent messages.
2. In German there is a difference between the informal "du" and formal "Sie" (https://www.dw.com/en/the-du-sie-dilemma-in-german/a-16494631).
   So the email address message could be "Bitte geben Sie eine gültige E-Mail-Adresse ein" or "Bitte gib eine gültige E-Mail-Adresse ein".
   At the moment the german validators file uses "Sie". But I manage a project, where we address the users by using the informal "du". So I have to replace all these validation messages.
   This could be avoided by using neutral messages like "This value is not a valid email address" / "Dieser Wert ist keine gültige E-Mail-Addresse".
   I think the same is true for some other languages (like tu/vous in french).