Fix Windows prerequisite detection on non-English systems

Request English DISM output so the existing feature-state parser works on localized Windows installations.

Fixes #155.

src/backend_win/prereq.c

@@ -20,7 +20,7 @@ BOOL prereq_is_feature_enabled(const wchar_t *feature_name)
     DWORD bytes_read = 0;
 
     _snwprintf_s(cmd, 512, _TRUNCATE,
-        L"C:\\Windows\\System32\\dism.exe /online /Get-FeatureInfo /FeatureName:%ls",
+        L"C:\\Windows\\System32\\dism.exe /English /online /Get-FeatureInfo /FeatureName:%ls",
         feature_name);
 
     ZeroMemory(&sa, sizeof(sa));