Looks like RSAT only supports those 19 language packs:
<assemblyIdentity name="Microsoft-Windows-WindowsFoundation-LanguagePack-Package" version="6.3.0.0" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="fr-FR" buildType="release"></assemblyIdentity>
<assemblyIdentity name="Microsoft-Windows-WindowsFoundation-LanguagePack-Package" version="6.3.0.0" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="zh-HK" buildType="release"></assemblyIdentity>
<assemblyIdentity name="Microsoft-Windows-WindowsFoundation-LanguagePack-Package" version="6.3.0.0" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="ja-JP" buildType="release"></assemblyIdentity>
<assemblyIdentity name="Microsoft-Windows-WindowsFoundation-LanguagePack-Package" version="6.3.0.0" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="ko-KR" buildType="release"></assemblyIdentity>
<assemblyIdentity name="Microsoft-Windows-WindowsFoundation-LanguagePack-Package" version="6.3.0.0" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="pt-BR" buildType="release"></assemblyIdentity>
<assemblyIdentity name="Microsoft-Windows-WindowsFoundation-LanguagePack-Package" version="6.3.0.0" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="zh-CN" buildType="release"></assemblyIdentity>
<assemblyIdentity name="Microsoft-Windows-WindowsFoundation-LanguagePack-Package" version="6.3.0.0" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="pt-PT" buildType="release"></assemblyIdentity>
<assemblyIdentity name="Microsoft-Windows-WindowsFoundation-LanguagePack-Package" version="6.3.0.0" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="pl-PL" buildType="release"></assemblyIdentity>
<assemblyIdentity name="Microsoft-Windows-WindowsFoundation-LanguagePack-Package" version="6.3.0.0" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="en-US" buildType="release"></assemblyIdentity>
<assemblyIdentity name="Microsoft-Windows-WindowsFoundation-LanguagePack-Package" version="6.3.0.0" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="tr-TR" buildType="release"></assemblyIdentity>
<assemblyIdentity name="Microsoft-Windows-WindowsFoundation-LanguagePack-Package" version="6.3.0.0" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="nl-NL" buildType="release"></assemblyIdentity>
<assemblyIdentity name="Microsoft-Windows-WindowsFoundation-LanguagePack-Package" version="6.3.0.0" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="sv-SE" buildType="release"></assemblyIdentity>
<assemblyIdentity name="Microsoft-Windows-WindowsFoundation-LanguagePack-Package" version="6.3.0.0" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="de-DE" buildType="release"></assemblyIdentity>
<assemblyIdentity name="Microsoft-Windows-WindowsFoundation-LanguagePack-Package" version="6.3.0.0" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="es-ES" buildType="release"></assemblyIdentity>
<assemblyIdentity name="Microsoft-Windows-WindowsFoundation-LanguagePack-Package" version="6.3.0.0" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="ru-RU" buildType="release"></assemblyIdentity>
<assemblyIdentity name="Microsoft-Windows-WindowsFoundation-LanguagePack-Package" version="6.3.0.0" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="hu-HU" buildType="release"></assemblyIdentity>
<assemblyIdentity name="Microsoft-Windows-WindowsFoundation-LanguagePack-Package" version="6.3.0.0" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="zh-TW" buildType="release"></assemblyIdentity>
<assemblyIdentity name="Microsoft-Windows-WindowsFoundation-LanguagePack-Package" version="6.3.0.0" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="it-IT" buildType="release"></assemblyIdentity>
<assemblyIdentity name="Microsoft-Windows-WindowsFoundation-LanguagePack-Package" version="6.3.0.0" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="cs-CZ" buildType="release"></assemblyIdentity>
You can add EN-US and easily download the language pack:
Using the language you want
http://blogs.msdn.com/b/b8/archive/2012/02/21/using-the-language-you-want.aspx