1. One URL per language
/our-rooms/ in the site's language, /de/unsere-zimmer/ in German. Each version has its own URL, and it never changes. Google files each language separately, and shows it to people searching in that language.
2. A page Google can read
The translation must be in the page the server sends, not added afterwards by the visitor's browser. Google can't see a translation done in the browser: as far as it's concerned, your site only exists in its original language.
3. Tags that link the languages together
Each page tells Google where its other versions are (the hreflangtags). Without them, Google may show the original page to a visitor searching in another language.
4. Titles and descriptions translated too
The title and description shown in search results are what make people click, or not. They must be translated like the rest of the page.
5. Nothing changes for the original version
The original URLs stay the same, and so does the content. What works today keeps working.