useTranslation

const { t, i18n } = useTranslation();
return <button>{t("checkout.pay")}</button>;

t(key, options?) - resolve a translation for the active locale; supports interpolation and plurals. When the FALC plainLanguage mode is on, t() returns the plain_language overlay for keys that have one.
<Trans i18nKey="..."> - rich-JSX translation.
i18n - the instance (see A11y accessors and setLocale / dir / setPlainLanguage).