Html5 required attribute adds normally the error message "Please fill out this field." and it can easily be changed with the following code
oninvalid="setCustomValidity('Custom text in another language..')"
oninput="setCustomValidity('')"
However, I have noticed that when I use inputs with type email I still get messages such as "Please enter a part following '@." and "Please include an '@' in the email address", how can I change them?
The custom error message comes from the element's title, or by using setCustomValidity, as you said.
But message like "Please include an '@' in the email address" come from the type="email" attribute. There are similar error messages when there is a required attribute, for example. So these messages come from the browser, and will be in the user's browser language. I wouldn't worry too much about this, as the browser will already be in a language the user understands, and as they are native to the browser, users will get used to these standard error messages.