If it's only Hong Kong people who report this, I suspect they're actually using some English operating systems / browsers, considering English is another official language there (is there any Cantonese operating system?)... Obviously they will see mixed content if they don't configure their browsers which send Accept-Language: en by default.

Geolocation should be used after Accept-Language, because users can configure Accept-Language "easily" but not their IP addresses. Plus, many people from the mainland China tend to use some proxies which break geolocationing.

I remember content served from /wiki/ can be language-converted; was it changed (again) recently? This would also benefit link sharing, so recipients see pages in their preferred variants, instead of sender's preferred one.

+1 to increasing the selector visibility
+1 to improving conversion intelligence for content served from /wiki/ , to support link sharing

Trusting Accept-Language has certain problems for zh-HK (and presumably also zh-SG, zh-MY and zh-MO):

• Many users/devices have only en/zh-TW set.
• Some platforms expose no way to set zh-HK (e.g. Chrome desktop and some versions of Android)
• Other platforms or expose no way to set zh-TW as a fallback locale (e.g. Chrome on Android)
• Some (non-Wikimedia) websites/apps incorrectly serve Simplified Chinese or even English for zh-HK (but not zh-TW)

So it's not a straightforward matter to push users to prepend zh-HK to Accept-Language.

Theoretically we could prioritise Accept-Language if it specifies a different script to the geolocation (e.g. a Simplified Chinese request from Hong Kong), but otherwise prioritise the geolocation. In pseudo-code:

$accept_script = guess script from accept-language ('Hans' or 'Hant' or null)
$geo_script = guess script from geolocation ('Hans' or 'Hant' or null)
if $accept_script != null and $accept_script != $geo_script:
    guess variant from accept-language
else:
    guess variant from geolocation

If neither Accept-Language nor the geolocation give any clue as to variant, a highly visible callout nudging the user to select a variant would seem sensible. (Anecdotally I've seen people in Hong Kong struggling through the mixed text that can appear right now - this must happen to overseas Chinese readers too).

This should be the actual tag adding example