OK..I have created a project for Custom Modules and I set out to Localize it.

Here is the structure

ViewSampleModule.ascx

EditSampleModule.ascx

Settings.ascx

SampleCtl.ascx  ..====> Its resx file is SampleCtl.ascx.resx placed under App_LocalResources

Now I have following HTML on both SampleCtl.ascx and ViewSampleModule.ascx

            <asp:Button ID="cnclBtn" runat="server" resourcekey="cnclBtn"/>

            <dnn:label id="Label1" runat="server" ControlName="txEnter"></dnn:label>

In SampleCtl.ascx file they showup empty whereas in ViewSampleModule.ascx it works just fine.

The entries in both the resx files are same,

  <data name="cnclBtn.Text" xml:space="preserve">
    <value>Cancel</value>
  </data>
  <data name="Label1.Help" xml:space="preserve">
    <value>Enter User Id</value>
  </data>
  <data name="Label1.Text" xml:space="preserve">
    <value>To</value>
  </data>

Any suggestions on how to get this working

Sebastian Leupold wrote Did you check out the Module Developers Localization Guide, available from Resources > Dócumentation > Downloads page in menu above?

I was able to resolve my issue as below.

I was using "SampleCtl.ascx" inside ViewModule.ascx..and The LocalResourceFIle is having a  value of "ID" for SampleCtl.ascx.cs at runtime, which never existed in the system.

So in OnInit() inside SampleCtl.ascx. I am explicitly setting LocalResourceFile to "SampleCtl.ascx.rex"