@Gil2599 said:
I have been testing using my desktop PC, Widows 11, and Chrome.
However, the request above randomly began working for me this morning.

Another that also does not work for me is:

https://api.themoviedb.org/3/movie/now_playing?api_key=####&page=1&language=zh-US&region=US

But the same request for the upcoming movie list works fine:

https://api.themoviedb.org/3/movie/upcoming?api_key=####&page=1&language=zh-US&region=US

As far as I know Mandarin, zh, it is not an Official Language spoken in the United States.
Only the ISO code of Official Languages spoken in a given country can be coupled to the ISO code of a country.
Therefore this combination of it, & language = zh-us or & language = zh-US, does not exist.
The parameter &language= specifies in which language will be the response data, such as the title or overview.
In the case of your two examples of API requests, was the response data that worked, was in Mandarin or came in English?

See some examples of countries with more than one official language.
A Country with Multiple Languages

CA  Canada 2 (en+fr)
CN  China 2 (cn+zh)
IN  India 5 (hi+kn+ml+ta+te)
NO  Norway 2 (nb+no)
ES  Spain 4 (ca+es+eu+gl)

And some examples of official spoken languages in more than one country.
A Language with Multiple Countries

ar  Arabic 2 (AE+SA)  
en  English 6 (AU+CA+GB+IE+NZ+US)   
fr  French 2 (CA+FR)
de  German 3 (AT+CH+DE)
ms  Malay 2 (MY+SG)
zh  Mandarin 4 (CN+HK+SG+TW)
pt  Portuguese 2 (BR+PT)
es  Spanish 2 (ES+MX)

This seems to be the same problem

https://www.themoviedb.org/talk/654435679cc67b00ab45ad86

"now_playing" and "upcoming" returning no results when using "language" and "region" parameters.

I’ll take a look at this problem this week. Something seems up but it looks to be quite inconsistent, as it does work sometimes but obviously isn’t other times.

@travisbell said:

I’ll take a look at this problem this week. Something seems up but it looks to be quite inconsistent, as it does work sometimes but obviously isn’t other times.

Thank you, I appreciate it. It seems like removing the region parameter makes it work regardless of language parameter value.

Can you give it a try now? I made a few changes that I suspect may have been causing this but I am not 100% sure.