a simpler and safer method than messing with the registry would be
central admin -> upgrade and migration -> Check product and patch installation status.
you will see a whole list on installs and the language like
Microsoft SharePoint Portal English Language Pack
english langauge is 1033 regardless of it being us or uk (1033), also in 12/14/15 hive youll see 1033 folder and any other number relating to other languages :).
to your question:
sharepoint looks in hive files under:
2007
c:\program files\common files\microsoft shared\web server extensions\12\resources
2010
c:\program files\common files\microsoft shared\web server extensions\14\resources
2013
c:\program files\common files\microsoft shared\web server extensions\15\resources
1033 is for english as noted... doesnt matter if its uk or usa but in the resoures files it would be under en-us as its an american product ;) ..... also you need to check what your SQL database server is setup as (uk or us) as the conflict could be happning there!?
could you post the full error in your question?
EDIT
following on from the error you posted:
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\Template\Features\MyWebPart_MyPart WebPart\Resources\Resources.en-US.resx
you have a custom webpart called MyWebPart_MyPart?
if you do than you need to open it up in visual studios and it should contain some resource files (resex). It looks like it hasnt been deployed during the wsp deployment to:
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\Template\Features\MyWebPart_MyPart WebPart\Resources
to make sure goto sharepoint server and goto the location above and look for the file Resources.en-US.resx
if its not there than the webpart that contains the feature is not adding the resource files properly.... either sort it out so that it adds the resource files on deployment or add the files manually.
adding manually goto the webpart project location where the project is stored and look for the resex file... copy them and past them in:
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\Template\Features\MyWebPart_MyPart WebPart\Resources
or add it again to visual studio:
1.If the SharePoint solution is not already open in Visual Studio, open the solution.
2.In Solution Explorer, open the shortcut menu for the name of a feature under the Features node, and then choose Add Feature Resource.
This step adds a resource file to the feature in the format,
ResourceFileName.culture.resx, such as, Feature1.en-US.resx.
3.When the Resource Editor opens in Visual Studio, add resources to the resource file.
http://msdn.microsoft.com/en-us/library/ee231537.aspx
now the key to your error is that its refering to the default resource (fallback file)
To specify default resource files in SharePoint projects that you
develop in Visual Studio, choose Invariant Language (Invariant
Country) in the culture list of the Add Resource dialog box when you
add a resource file.
http://msdn.microsoft.com/en-us/library/ee696750.aspx
to do this you do as stated above. This time select Invariant Country rather than englist (US).
so rule of thumb. when creating resource you need two... one is default (no country) which would be resources.resx and the other resources.en-us.resx
see if this works!
if it still complains that you need to add it into the feature.
within visual studio within the project goto feature folder, within the feature folder select the name of the feature... by default its called Feature1... double click and let the page load and then select properties (right hand side).... within the properties tab for the feature you should see 'default resource file', within the box on the right put in the name of the default resource file that you made!
now rebuild and deploy and everything should work!