function GetUILanguage: Integer;

Returns the language identifier (LANGID) of the current user's UI language, which is either the language of Windows itself, or in the case of a MUI edition of Windows, the user interface language chosen in Control Panel's Regional Options. Returns 0 if the function fails (unlikely).

Refer to the list of valid language identifiers on MSDN.

begin
  if GetUILanguage = $0409 then
  begin
    // UI language is English (United States)
  end;

  // You can use "and $3FF" to extract the primary language identifier
  if GetUILanguage and $3FF = $09 then
  begin
    // Matches any variant of English
  end;
end;