Good idea so I have had a go. It would be great if you could help me through getting this submitted if you have the experience to help?

I modified web.config:

<section name="localization" requirePermission="false" type="DotNetNuke.Framework.Providers.ProviderConfigurationHandler, DotNetNuke"/>

Then later in the dotnetnuke section added:

    <localization defaultProvider="LocalizationProvider">
      <providers>
        <clear/>
        <add name="LocalizationProvider" type="DotNetNuke.Services.Localization.LocalizationProvider, DotNetNuke" providerPath="~\Library\Services\Localization\LocalizationProvider\"/>
      </providers>
    </localization>

Then in DotNetNukeHttpApplication:

            ComponentFactory.InstallComponents(new ProviderInstaller("localization", typeof(ILocalizationProvider))); 

I didn't touch the LocalizationProvider as I didn't grock what this does:

    public class LocalizationProvider : ComponentBase<ILocalizationProvider, LocalizationProvider>, ILocalizationProvider

Finally I created a ILocalzationProvider that simply wraps the original:

    public class CustomLocalizationProvider : ILocalizationProvider
    {
        private readonly ILocalizationProvider localizationProvider;

        public CustomLocalizationProvider()
        {
            localizationProvider = new LocalizationProvider();
        }

        #region ILocalizationProvider Members

        public string GetString(string key, string resourceFileRoot)
        {

            return localizationProvider.GetString(key, resourceFileRoot);
        }

        public string GetString(string key, string resourceFileRoot, string language)
        {
            return localizationProvider.GetString(key, resourceFileRoot, language);
        }

        public string GetString(string key, string resourceFileRoot, string language, PortalSettings portalSettings)
        {
            return localizationProvider.GetString(key, resourceFileRoot, language, portalSettings);
        }

        public string GetString(string key, string resourceFileRoot, string language, PortalSettings portalSettings,
                                bool disableShowMissingKeys)
        {
            return localizationProvider.GetString(key, resourceFileRoot, language, portalSettings,
                                                  disableShowMissingKeys);
        }

        #endregion
    }

Then to test I setup the config to use it:

    <localization defaultProvider="CustomLocalizationProvider">
      <providers>
        <clear/>
        <add name="LocalizationProvider" type="DotNetNuke.Services.Localization.LocalizationProvider, DotNetNuke" providerPath="~\Library\Services\Localization\LocalizationProvider\"/>
        <add name="CustomLocalizationProvider" type="MyCompany.Services.Localization.CustomLocalizationProvider, Enhesa.Services.Localization"/>
      </providers>
    </localization>

Then I ran the website in debug and low and behold I found my CustomLocalizationProvider was indeed intercepting all the GetString() methods.

Is there something I missed? It felt a bit too easy:-)  I wondered if the LocalizationProvider should be moved to a providers folder?