Resource translation in custom WebPart failure

Some resources of my project aren't translated properly. Wanted behavior is resource displayed in both French and English. Default site's language is English.

When I force my web browser to display content in French it works fine:

but in English it fails:

What is wrong with my code?

CustomWebPartManager manager = new CustomWebPartManager("MSContentEditor.dwp", web);
manager.AddWebPartToPage(welcomePageUrl, "MainZone", 0);
manager.SetWebPartPropertyXml("Content", resM.GetLocalized("searchWelcomeMsg"));

Following is GetLocalized code:

public string GetLocalized(string strId)
    {
        return SPUtility.GetLocalizedString("$Resources:" + strId, _resxName, _currentLang);
    }

All resources files are deployed (XXXX-SearchCenter.en-US.resx and XXXX-SearchCenter.fr-fr.resx).

Translation only fails in this custom web part and in a custom menu node definition.