I find that this problem occurs only if your potal language is different than en-US (in my case fr-FR) with DotNetNuke 4.9.0

If find that in Mail.vb infunction SendMail use user.Profile.PreferredLocale
with is for exemple en-US (US in uppercase)

passed to Localize.GetSystemMessage(locale, settings, subject, user, Localize.GlobalResourceFile, custom, "", settings.AdministratorId)

witch indirect call Localization.GetResource(ByVal ResourceFileRoot As String, ByVal objPortalSettings As PortalSettings, ByVal strLanguage As String) As Hashtable

In this function find :

If strLanguage Is Nothing Then
  userLanguage = Thread.CurrentThread.CurrentCulture.ToString.ToLower
Else
  userLanguage = strLanguage
End If

witch mean userLangage not in lower case

I change to :

If strLanguage Is Nothing Then
  userLanguage = Thread.CurrentThread.CurrentCulture.ToString.ToLower
Else
  userLanguage = strLanguage.ToLower
End If
 

and everything is ok now

Forge projects : Open Url Rewriter - Open Content - Open Form

IMHO it would be best not to do a comparison of Culture Keys case sensitive, in GetResource or the cultureInfoComparer.