As the title states, it’s possible to get the profile page activity feed to load the wrong language feed if you try to load two separate tabs with different languages.
I’m able to consistently reproduce the behaviour by opening the “viewing [Language]” drop down, middle-clicking to open another language in a new tab, then quickly clicking on a different language in the original tab. The new tab will load the page under the page routing of whatever language you clicked on, but will seemingly contain the activity info for whichever language was last requested (the one you click on in the original tab).
I’ve tested the behaviour across a few different user pages now, all of which do the same thing. I’m currently on Firefox, haven’t tested it on anything else.
Here’s a picture of the result: