This is one of the great parts of the web: you can tag every element with the global lang attribute and have things work the way you expect.
For example, you can have CSS generate the appropriate quotation marks even in nested contexts so you can painlessly use <q> tags to markup scholarly articles even if the site itself is translated and thus would have different nested quotation marks for, say, the French version embedding an English quote including a French quote or vice versa.
In your Reddit example, the top level page should be in the user’s preferred site language with individual posts or other elements using author’s language: <html lang=en>…<div lang=fr>