Hi Stefan, 

It looks like you are having the same issue as at this forum post.  http://ko.infragistics.com/community/forums/p/78241/395684.aspx

I believe you are asking about two of the Shared controls, XamComboEditor and XamScheduleView, not WPF only controls.  (I believe you are referencing the XamComboEditor which uses the Infragistics.Controls.Editors namespace.  There is also a WPF Only xamComboEditor that uses the Infragistics.Windows.Editor namespace.) 

Customizing Resource strings is for the WPF only controls.  There are links here for the WPF Only supported controls, which it sounds like you have been using successfully.  http://help.infragistics.com/Help/Doc/WPF/2013.2/CLR4.0/html/GeneralConcepts_Customizing_Resource_Strings_for_WPF_Only_control.html

For the Shared controls you will need to use the other technique of modifying resource files.  You can find the controls' resource files in the following location:

C:\Program Files (x86)\Infragistics\2013.2\WPF\CLR4.0\Bin 

The technique is described here in the section Modifying the Resource Files:

http://help.infragistics.com/Help/Doc/WPF/2013.2/CLR4.0/html/Developers_Guide_Customizing_Resource_Strings.html 

I would also recommend that you add your idea concerning using Customizing resource strings for the Shared controls as a new product idea at our product idea site<http://ideas.infragistics.com>.

The Product Ideas site allows you to suggest new product features, new functionalities in existing products and controls, and even request new controls and products altogether. Members of the Infragistics Community can then vote for the features and suggestions they want to see added to the products, resulting in the most popular features bubbling up to the top. When planning new releases, our Product Management team will look at the most popular features and use your feedback to prioritize upcoming work.

This new Infragistics Product Ideas site is simple to use and we encourage you to take a look at it today:

    1. Log into the Infragistics Product Idea site at http://ideas.infragistics.com (creating a new login if needed).

    2. Navigate to the product / platform channel of your choice (e.g. WPF, Windows Forms, ASP.NET, HTML5 / Ignite UI, iOS / NucliOS, etc.)

    3.  Add your product idea and be sure to be specific and provide as much detail as possible. Explain the context in which a feature would be used, why it is needed, why it can’t be accomplished today, and who would benefit from it. You can even add screenshots to build a stronger case. Remember that for your suggestion to be successful, you need other members of the community to vote for it. Be convincing! 

The Product Idea site puts you in the driver’s seat and you can even track the progress of your ideas at any time, see how many votes it got, read comments from other developers in the community, and see if someone from the product team has additional questions for you. 

Please let me know if you have any questions.