E.g. take class A of students from one culture and class B of students from another culture, and assume the students all feel the same level of moderate anger, but the two cultures have different "anger display rules" as they're called.

The detector might judge all the students from one class as angry, but none from the other. Aggregating students into classes doesn't fix anything.

Similarly, if you took some students who have a certain expression when they're happy, and other students who have the same expression when they're afraid, and mixed them together in the same class, then whatever emotion the detector is interpreting the expression to mean is going to be triggered by completely unrelated things, and the signal won't be meaningful at all.

Yes, there are differences - especially in identification of intensity. But in general detection is pretty reliable across cultures.

Eg:

The results show that the overall accuracy of emotional expression recognition by Indian participants was high and very similar to the ratings from Dutch participants. However, there were significant cross-cultural differences in classification of emotion categories and their corresponding parameters. Indians rated certain expressions comparatively more genuine, higher in valence, and less intense in comparison to original Radboud ratings. The misclassifications/ confusion for specific emotional categories differed across the two cultures indicating subtle but significant differences between the cultures.

https://journals.plos.org/plosone/article?id=10.1371/journal...

Unfortunately that's simply not true. Your example merely shows reliabiliy between two countries, not all countries. You can research, for example, how Japanese people tend to smile when angry, sad, or embarrassed, in total contrast to Westerners.

This is one of the classic "extreme" examples, but it demonstrates how my point holds -- errors are not random but are highly correlated with the culture -- not to mention the subculture, the individual (the person who hides all emotion when angry), etc.