To allow opening files on Unicode paths on Windpws, expect passed 8-bit
strings to be UTF-8-encoded, and convert them to wchar_t. Fallback to
ACP strings for backward compatibility.

diff --git a/src/Numbertext.cxx b/src/Numbertext.cxx
--- a/src/Numbertext.cxx
+++ b/src/Numbertext.cxx
@@ -27,7 +27,11 @@
 bool readfile(const std::string& filename, std::wstring& result)
 {
 #ifdef _WIN32
-    std::ifstream ifs(filename);
+    // First try to convert from UTF-8
+    std::ifstream ifs(Numbertext::string2wstring(filename));
+    // Fallback to ACP string for backward compatibility
+    if (ifs.fail())
+        ifs.open(filename);
     if (ifs.fail())
         return false;
     std::stringstream ss;