It's happen some times in my websites.
Is really strange, because sometimes just because I changed some dlls in my gadget I get this error. I never was able to fix without install again the web site.
Someone know whats happen and how can I fix it?
Check your xml-files to ensure that this isnt already covered by the localizationfiles in your project. Most likely, your FileXmlLocalizationProvider is broken, and thats why your reinstall solves the problem. Make sure your episerver.framework file contains the following;
<localization fallbackBehavior="Echo, MissingMessage, FallbackCulture" fallbackCulture="en">
<providers>
<add virtualPath="~/Your/Path/To/Translations" name="languageFiles" type="EPiServer.Framework.Localization.XmlResources.FileXmlLocalizationProvider, EPiServer.Framework" />
</providers>