Naturally, the translation field is filled in, but I think it does not intercept the empty-form.message variable
The message in question is displayed by the browser as part of HTML form validation. If the user’s browser is switched to Bulgarian, the error messages will be translated automatically. It is possible to force these messages to a different language, although I haven’t done this personally. Stack overflow has some suggestions.
The translation options in the sidebar are for the popup that Smart Forms displays when you submit the form (that contains the recaptcha) and they don’t affect the browser validation.
You can disable the browser default feedback tooltips and use bootstraps custom-tooltips or custom-feedback in Bulgarian for your forms