Adding another input language always changes keyboard layout?

Here is a modified layout you can try to see if it helps solve the issue your raised. If you are using a different qwertz layout I can easily post a modified version of that.

You put the .bundle file in Library/Keyboard Layouts, logout/login, and then activate it in system settings > keyboard > input sources. It should be found under the English category.

Input Sources differ primarily in terms of the keyboard layout, but it happens they are also tagged by language, so that they will appear in the correct catagory when users look for them in the settings. Normally that tag does not affect the way apps behave, but you have discovered an example where it does matter (MS Word is another one). A possible fix for this is to make a custom keyboard layout with the language tag modified using Ukelele. I will post an "English" German layout for you to try a little later.

This is actually more complicated than it sounds. There are three issues to deal with:

1) Localization. All Mac software can be localized to run in different languages. This affects the text displayed, sometimes the direction of the text, and perhaps other aspects as well.

2) Keyboard layout. Different places in the world have different keyboards. Often Apple devices have less regional variation than PC keyboards. For example, there is one English Mac keyboard in North America. But PCs tend to have a special "Canadian" keyboard layout.

3) Input method. This idea is harder to pinpoint. I'm at my Monterey computer right now and it has a different keyboard user interface. Plus, I don't speak German so I don't know what your display is telling you. But regardless, each keyboard may have a different input method for keys that don't physically appear on the keyboard. For example, there is a whole set of "dead keys" that can be used to type most European languages with characters like é, è, ß, ü, etc.

I find the "dead key" method easy to learn, intuitive, a very amenable to muscle memory. I can type most of them (les lettres français, au moins) without thinking. This method may be a feature of the default US keyboard. I'm not very familiar with other keyboards. At the other extreme is the press-and-hold iOS style pop-up. People who are only familiar with this method from iOS get very angry if you suggest other methods.

I say all of this because I'm not sure what you are seeing in those displays. Since the emoji viewer is considered a "keyboard thing", it might respond to changes in the keyboard. Or it might just be localization. Or it might be some unholy combination of both. I can tell you that in Monterey, in French localization, I can search for both "heart" and "coeur" and get the same emoji. When I search for "coeur", I do not get the playing cards whose names are defined only in English.

Furthermore, changes to the current localization and keyboard settings may not take effect right away. Usually, you have to restart to completely make the change. Often, you can get away with just logging out and back in. But the operating system is comprised of hundreds of separate processes and these won't adapt to a new localization until they are restarted. I honestly don't know about keyboard changes.

And finally, there is cultural hegemony. Apple is proud that their products are "designed in California". They ain't kiddin' there. The expectation is that you are an anglophone who might know one foreign language at best. If you aren't an anglophone, then they do have a global localization budget and you'll get a pretty good translation. But if you aren't anglophone, but code switch on a regular basis, that's a foreign concept to most Americans. You'll have to figure that out on your own. As an American who halfway speaks French, use the legacy US keyboard and it works pretty good for me. I'm not sure if it is possible to do the same from a non-English perspective.