Under HTML forms > your form > Messages you can set the success messages etc.
Tooltips like “this field is required” are HTML5 default broswer behavior and will show in whatever language the browser was installed.
To translate the complete form I suggest to make 1 form for each language.
Translation is not a default feature of WordPress, so the way it would be translated (besides just making one separate form for each language) is really up to whatever translation plugin you are using.
Hope that helps. If you have any questions, please let us know!