What is considered the best strategy for URLs of translated websites? Something I see happen frequently is:
http://example.com/english-slug.html
http://example.com/de/english-slug.html
http://example.com/fr/english-slug.html
This has the (minor) advantage that a user can manually switch to another language by modifying the URL. The disadvantage seems to be that the URL consists of a slug in the wrong language for every page not in the default language. That would cause a SEO penalty, I think.
The alternative would be to translate the slugs as well and optionally omit the language identifier as well:
http://example.com/english-slug.html
http://example.com/deutscher-slug.html
http://example.com/slug-francois.html
Some languages do not really lend themselves to be 'sluggified', such as Russian, Chinese and Arab. You'll end up with transliterations that make little sense.
I think you should use language tags and translated slugs:
http://example.com/en/hello-world
http://example.com/de/hallo-welt
Why language tags? To avoid collisions.
Sometimes you might want to have the same slug for different languages (for example, "team" in English as well as in German).
Why translated paths? For better usability.
Not all users understand the "default" language, and depending on its script, they might not even be able to type/remember/dictate the slug. (It doesn’t make sense in the first place to use human-readable slugs that only part of your users are able to understand.)
Yes, it would be a "(minor) advantage that a user can manually switch to another language by modifying the URL". But it’s unlikely that users or search engines would expect this to work (see my related answer on Webmasters). So you would gain little (URL hacking for advanced users) and lose much (bad usability for users using the non-default language).
That said, it would still be possible to enable this URL hacking feature in most cases: When users change the language tag de (in /de/hallo-welt) to en (/en/hallo-welt), you could check if it exists (if yes, show it), and if not, check if there exists a slug "hallo-welt" in any language, find its English translation and redirect to it (from /en/hallo-welt to /en/hello-world).