How can I easily find all languages a show has been dubbed into?
Translating web pages automatically with Google Translate°

This would mean that I would have to check (and be able to read) each […] page in each potentially interesting language.

Google Translate goes a long way in helping that.  Although unfortunately its translations of Asian languages can still be tricky to understand.
If you use the Google Chrome browser, it should automatically offer to translate foreign pages (but does not consistently do so for me).
If you want to test that, here's the Cantonese Wikipedia page for Pretty Cure:
https://zh-yue.wikipedia.org/wiki/Fresh_Precure!
If you use another browser, e.g. Mozilla Firefox, or Chrome doesn't offer the translation by itself:
Put this before the URL to translate:                               https://translate.google.com/translate?&amp;u=
Example: https://translate.google.com/translate?&amp;u=https://zh-yue.wikipedia.org/wiki/Fresh_Precure!

° Sorry this is not an answer to the question, but I find it so important, that I wanted it to be more visible.  (And then I ended up writing more than would fit into a comment.)  If the community objects, I will move this to a comment.
In relation to the question, the translation enables you to apply the useful approach, suggested by the asker, of consulting the anime's page on the Wikipedia(s) of your desired dub language(s).  (Or opening foreign Google results.)