Default localization XML file in different Culture

Question

We are currently using ASP.NET ZERO (ABP). Throughout the course of implementation the XML files with LocalizedTexts got bigger and bigger.

To manage LocalizedTexts we are using a build-in manager:

We use XML (file.xml) file with culture "en" and the LocalizedTexts are being added there. There are also other XML files with other cultures like "de" (file-de.xml) where some keys are added but some don't.

Problem occures when the site's language is set to "de", then the manager fetches all keys from "de" file (file-de.xml) and ultimately there is no way to apply translations to those LocalizedTexts which keys are in "en" (file.xml) but not in "de" (file-de.xml).

Is there a neat way to access all keys in the manager without adding them to multiple files simultanously?

I know we can fetch both and do a union:

var allDefaultStrings = source.GetAllStrings(CultureInfo.GetCultureInfo("en"));
var allStrings = source.GetAllStrings();
var union = allStrings.Union(allDefaultStrings, new LocalizedStringComparer());