Switching InDesign language resets all presets, shortcuts and workspaces — seriously?

Can you please explain why this still works like it’s 2003?

InDesign CS (that is, version 3) was released in 2003 - I'm pretty sure that it still works in this 2003-reminiscent way due to the fact that the initial InDesign UI locale-handling design decisions were made well before 2003. I will happily agree with you regarding how locale is best managed in contemporary apps, and I would love to see those kinds of changes in InDesign, but I'm not holding my breath. I'm sure that we'd see fewer locale-specific bugs, if the dev team had the time and resources to completely reengineer locale handling. 

At the very least, there should be a migration option — or shared user data between languages.

They've already recently implemented user preference export and import tools - they're in the File menu. Seems like user pref export/import could be modified to handle the case of "export Czech UI prefs & import them into en_US locale", if it can't handle it already.  I just exported my prefs and am looking at them in a text editor, and while some locale-specific identifiers are used, it looks like it shouldn't be hard at the developer's end to make user prefs locale-independent. Honestly, it doesn't look impossible to convert post-export. 

Just to add on: I would be grateful for a preference switch to keep the Preview switches on in so many dialog boxes! Everytime I have to troubleshoot, patch, or upgrade a version, they all are switched off again. 

Each language has its own presets folder. But you can copy the content of one folder copy to another. It will transfer all saved settings to the other languages. 

You’re right it’s a limitation, especially for multilingual users or anyone switching UI languages for collaboration or training.

Unfortunately, InDesign treats each language as a separate installation environment, which is why all user data (workspaces, shortcuts, preferences, etc.) gets stored in folders like:
~/Library/Preferences/Adobe InDesign/Version <xx>/<locale> (macOS)
or
%AppData%\Adobe\InDesign\Version <xx>\<locale> (Windows).

That means switching from cs_CZ to en_US doesn’t delete your presets - they’re just in the Czech folder. You can manually copy them across by duplicating the corresponding subfolders (for example, “Workspaces,” “Keyboard Shortcuts,” and “Print Presets”) from the Czech folder into the English one.

Quick steps:

Close InDesign.

Go to your old locale folder (e.g. cs_CZ).

Copy the folders for your custom items (like Workspaces, InDesign Shortcut Sets, PDF Presets, etc.).

Paste them into the new locale folder (en_US).

Relaunch InDesign.

It’s definitely something Adobe could handle more gracefully  a migration or shared profile option would make a huge difference.

You might also want to file it as a feature request on the Adobe InDesign UserVoice page it helps when feedback like this gets visibility from other professionals.

https://indesign.uservoice.com/forums/601021-adobe-indesign-feature-requests