Conclusion: give icon-only buttons (like a modal close X) a translated aria-label only. Don't add a title with the same text, and never put aria-hidden="true" on the focusable button.
Why not aria-label + title: when aria-label provides the name, browsers expose title as the accessible description, so the same text is exposed twice. In Chromium, aria-label="Close" title="Close" gives name "Close" and description "Close". NVDA with Chrome/Edge and VoiceOver can then say it twice, for example "Close, button, Close".
Why not title alone: it works as a fallback name, but the W3C guidance asks to avoid fallback naming, and the tooltip only appears on mouse hover, so keyboard and touch users never see it.
If a visible tooltip is needed: use a real tooltip shown on hover and focus, tied with aria-labelledby, not title.
