That key is not missing in the english resource file App_GlobalResources/Resource.resx
In Web.config, english is specified ias the default culture, if you changed it something else, you will get null reference exceptions because of incomplete translation of resource strings.
<globalization culture="auto:en-US"
uiCulture="auto:en-US"
requestEncoding="utf-8"
responseEncoding="utf-8"
fileEncoding="iso-8859-15"/>
The "auto" part of the setting tells it to use the users preferred language as specified in their browser settings. If the resources is not found for the preferred language, it falls back to the default. Therefore the default must be a complete translation with no missing keys for all resource files. Only the en-US resources are ever completely up to date, therefore you should leave en-US as the default.
Hope it helps,
Joe