For example, I see most of the Google's properties is using the
hl query string, e.g.
and compare the above URL with
From the SEO and usability perspective, which one is better?
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Here is the official documentation from Google about multilingual sites. The main point is:
Keep the content for each language on separate URLs.
As URL parameters tend to be ignored or scored down by search engines, the second URL should be considered better.