Google does place the language tag in the URL:
https://support.google.com/webmasters/answer/182192?hl=de
https://support.google.com/webmasters/answer/182192?hl=en
https://support.google.com/webmasters/answer/182192?hl=fr
But they don’t redirect the variant without the language tag to the variant with the language tag. Instead, they set the variant with the language tag (for the currently displayed language) as canonical. That way, only the variants with language tag get indexed (see example site: search).
It’s primarily a usability question if variants without language tag are needed in the first place, and if they should redirect to the variant with language tag, or if they should only have it as canonical URL. It affects the case where someone publishes a link:
- Should the link lead to the language-neutral version (so visitors following the link see the page immediately in their preferred language), or
- should the link lead to the same version the publisher saw (so visitors may have to use a language switcher, but it allows them to immediately see/verify quoted content)?
That said, even if Google itself wouldn’t use language tags in the URLs, it doesn’t matter what they do, only what they recommend:
Use different URLs for different language versions
Google recommends using different URLs for each language version of a page rather than using cookies or browser settings to adjust the content language on the page.