Hello Andrzej,

Thank you for sending us the project and the video demonstrating the observed behavior. I managed to reproduce the issue locally. Looking into the serialized code in the .Designer.cs file I noticed that for most of the controls there are calls similar to this one: 

((Telerik.WinControls.UI.RadCheckBoxElement)(this.radCheckBoxGroupRendererGroupInView.GetChildAt(0))).ToolTipText = resources.GetString("resource.ToolTipText1");

These code blocks get serialized after using the Element Hierarchy Editor inside the Visual Studio designer. As you are having a localizable user control, the resource manager in Visual Studio will try to write this setting in the resource file for the given language. On our side we don't have control on how these settings are written inside the resource files. This process is managed by the ComponentResourceManager class which is responsible for serializing, deserializing and mapping resources and settings for a given component.

In your actual project besides the TooltipText some other properries for child elements get serialized, namely the Text and Image properties of the checkboxes and buttons. As the resource manager has issues finding the correct settings you end up with mixed values sometimes. The good news is that this type of serialization is not necessary for the Text and Image properties as they get serialized on control level. What actually happens is that  the Text and Image they get serialized twice, once for the control - with correct values and later for the child element - with wrong values. For you actual scenario this seems to be a side effect of working with the ToolTipText through the Element Hierarchy Editor in the Designer. So I can suggest the following:

1. Remove all the .GetChildAt(0) entries in your designer file on controls that you have localizable properties. You can do it by hand. Most of the properties that are set there are also set to the control and they will be properly deserialized and serialized when changing the Language property.
2. For now don't open the Element Hierarchy Editor to set the ToolTipText property. Otherwise this will force double serialization of the problematic Text and Image properties and their settings will be mixed. Instead set the ToolTipText progrmmatically, you can follow the example demonstrated here: WinForms Buttons Button Tooltips - Telerik UI for WinForms.

I have updated your project to remove the GetChildAt entries in the designer file and you can find it attached.

We are already investigating a solution which will prevent the double serialization of the Text and Image properties for buttons and labels. I have approved the bug report and I have updated your Telerik points.

I hope this will help. Let us know if you need further assistance.

Regards,
Hristo
Progress Telerik

Hi Andrzej,

Thank you for the provided video and project. We will need more time to perform some tests on our side. We will contact you again as soon as we have more information regarding the reported behavior or questions.

Regards,
Dinko | Tech Support Engineer
Progress Telerik

Hi Andrzej,

We are currently looking into this bug report and we will need some additional information. Initially we isolated some problems related to the UI when opening the Designer on High DPI. They seem unrelated to your actual issue with the Language property and the culture-specific resources. We will need your help to dig deeper into the root cause of the behavior observed on your end.

If I understand correclty you are having a user control with culture-aware localization implemented through the Language property resulting in culture-specific resource files. After changing the Language of the user control and after adding a control usually by copying you end up with properties from one control being set to another. I tested this setup inside the project you provided but I could not observe the issue. On my end I don't see control properties being changed and set to other controls. I've used the project and screenshots you provided and I am attaching a short video demonstrating the result on my end. I also tested other ways adding controls and changing settings but I could not observe issue after changing the user control's Language. Can you please provide us with exact repro steps and we will try and replicate the setup locally? Additionally please check how a similar scenario will behave in a different project, if possible please also test with VS2022 or VS2026 and let us know of the results.

Genrally speaking it is Visual Studio that is managing how properties get serialized inside the resources files. We don't have logic inside our controls that is altering the default behavior. Changing the Language property of a form or a user control will cause the designer to reload all the UI and it will reintantiate all controls. In the process it will also read the .resx file associated with the specified Language. It seems that on your end it is mixing some of the control settings. We will do our best to assist you but please provide us with more details and the exact repro steps so that we can investigate on our end. 

Looking forward to hearing from you.

Regards,
Hristo
Progress Telerik

Hello Andrzej,

After reviewing the backlog item, it has not yet been added to our current plan for fixing. I understand that this behavior is essential for your application. I have already increased the priority of the feedback item and forwarded your last post to our development team, along with additional details about its impact on your side. Please accept my apology for the inconvenience caused by this situation.

As soon as I have more information about the current status of the item, I will reach out again so you can stay informed.

Regards,
Dinko | Tech Support Engineer
Progress Telerik

Hello, Andrzej,

Thank you for providing a project that replicates the issue. I was also able to observe a broken design view when changing the language of the user control. At this moment, I am not able to suggest a workaround to prevent this behavior.

I converted this ticket into a bug report so our developers can fix it. You can see it in our feedback portal here: Design view is destroyed after switching between Default (English) and Polish languages

I updated your Telerik Points for this report.

Regards,
Nadya | Tech Support Engineer
Progress Telerik