The reference uses the words "Consider" and "when possible", which is not as clear as other suggestions Google make. Instructions are crystal clear for other on-page techniques, such as hreflang.
As a power user who works with clients in multiple languages, I frequently switch between languages using the URL, like going from https://support.google.com/webmasters/answer/76329?hl=en to https://support.google.com/webmasters/answer/76329?hl=fr. This wouldn't be possible if the URL was https://support.google.com/webmasters/answer/keep-a-simple-url-structure. For this particular use, I would argue the former are more "user-friendly" than the latter!
More and more the URL is becoming a relic of the past. Sitename and Breadcrumbs are replacing it in SERPs. Browsers on mobile hide it by default. There is no URL bar in recent in-app browsers (Twitter, Facebook, LinkedIn).
On the hand, it has been said in the past that keywords in URLs help search engines understand the context of a link when there is no anchor text.
A few things to consider:
- The need to create 301 redirects and the risk of losing trafic
- The impact on on-site SEO (hreflang, canonicals, sitemaps, internal links, etc.)
- The qualitative impact (do your users expect this feature? do visitors expect this feature?)
- Most importantly, the fact that it's probably a low priority optimization!
- If at all possible, consider running an experiment.
Hope this helps! I left out a clear answer on purpose - because I don't have one. 
