It seems like contrary to Lemmy-UI, Photon does not display the user supplied post language anywhere and also has no way to set the post language in the UI. Nor does it have the profile setting to set language visibility.

I generally think that this feature in Lemmy-ui causes more harm than good due to the various ways in which people accidentally deselect "undetermined" as a language option, or how community mods accidentally prevent people from posting due to strict language restrictions... and a bunch of other related bugs that caused me to abandon this feature.

But having no representation at all seems like a bad idea given so much of Lemmy uses that feature and it also seems like Photon does hide wrongly language tagged posts if people misconfigure their profile settings in Lemmy-ui?

I can't confirm the latter, but this report https://slrpnk.net/post/14769073 sounds a lot like that is what is happening.