This is different from the editor's own language setting (the one you pick for yourself while building). That only changes what YOU see while working, it never changes what a visitor or end user sees. This feature lets a specific field show different text to different people, based on their own browser or device language.
Where it's available
One field per project type today: Website's and Web Application's Heading block, Desktop's Label block, Mobile's Text block, and a Form Builder field's own Question text. More fields will get this over time, it isn't on every text field yet.
Adding a translation
Open that block/field's own settings, under Other languages (optional) click + Add language, then enter a language code (en, de, fr, and so on, these are standard two-letter codes, not full language names) and the translated text for it. Add as many as you want, one row per language.
What a visitor actually sees
A visitor whose own browser or device language matches one of your added codes sees that version instead. Everyone else, including you while editing, still sees the field's own original text, which never goes away and always stays the fallback. There's no language switcher added anywhere, this is automatic based on the visitor's own settings, not something they pick on your page.
Tip: A code like de matches any German speaking visitor regardless of region (Germany, Austria, Switzerland). You don't need separate de-DE/de-AT entries unless you specifically want to say something different to each one.