diff --git a/toonz/sources/common/tsystem/tfilepath_io.cpp b/toonz/sources/common/tsystem/tfilepath_io.cpp
index 3c9d0fd..45d4df9 100644
--- a/toonz/sources/common/tsystem/tfilepath_io.cpp
+++ b/toonz/sources/common/tsystem/tfilepath_io.cpp
@@ -31,8 +31,12 @@ FILE *fopen(const TFilePath &fp, string mode) {
   return pFile;
 }
 
-Tifstream::Tifstream(const TFilePath &fp)
-    : ifstream(m_file = fopen(fp, "rb")) {}
+Tifstream::Tifstream(const TFilePath &fp) : ifstream(m_file = fopen(fp, "rb")) {
+  // manually set the fail bit here, since constructor ifstream::ifstream(FILE*)
+  // does not so even if the argument is null pointer.
+  // the state will be refered in "TIStream::operator bool()" ( in tstream.cpp )
+  if (!m_file) setstate(failbit);
+}
 
 Tifstream::~Tifstream() {
   if (m_file) {
@@ -47,7 +51,12 @@ void Tifstream::close() {
 }
 
 Tofstream::Tofstream(const TFilePath &fp, bool append_existing)
-    : ofstream(m_file = fopen(fp, append_existing ? "ab" : "wb")) {}
+    : ofstream(m_file = fopen(fp, append_existing ? "ab" : "wb")) {
+  // manually set the fail bit here, since constructor ofstream::ofstream(FILE*)
+  // does not so even if the argument is null pointer.
+  // the state will be refered in "TOStream::operator bool()" ( in tstream.cpp )
+  if (!m_file) setstate(failbit);
+}
 
 Tofstream::~Tofstream() {
   if (m_file) {
diff --git a/toonz/sources/toonzqt/fxsettings.cpp b/toonz/sources/toonzqt/fxsettings.cpp
index b72f579..571480e 100644
--- a/toonz/sources/toonzqt/fxsettings.cpp
+++ b/toonz/sources/toonzqt/fxsettings.cpp
@@ -15,6 +15,7 @@
 #include "toutputproperties.h"
 #include "pluginhost.h"
 #include "tenv.h"
+#include "tsystem.h"
 
 #include "toonz/tcamera.h"
 #include "toonz/toonzfolders.h"
@@ -822,6 +823,7 @@ void ParamsPageSet::createControls(const TFxP &fx, int index) {
 
   TFilePath fp = ToonzFolder::getProfileFolder() + "layouts" + "fxs" +
                  (fx->getFxType() + ".xml");
+
   TIStream is(fp);
   if (!is) return;
 
@@ -904,8 +906,18 @@ void ParamsPageSet::openHelpFile() {
   // if (m_helpCommand != "")
   //	commandString += m_helpCommand + " ";
 
-  TFilePath helpFp =
-      TEnv::getStuffDir() + TFilePath("doc") + TFilePath(m_helpFilePath);
+  // Get UI language as set in "Preferences"
+  QString currentLanguage = Preferences::instance()->getCurrentLanguage();
+  std::string helpDocLang = currentLanguage.toStdString();
+
+  // Assume associated language subdir exists
+  TFilePath helpFp = TEnv::getStuffDir() + "doc" + helpDocLang + m_helpFilePath;
+
+  // Verify subdir exists; if not, default to standard doc dir
+  if (!TFileStatus(helpFp).doesExist()) {
+    helpFp = TEnv::getStuffDir() + "doc" + m_helpFilePath;
+  }
+
   // commandString +=
   // QString::fromStdWString(helpFp.getWideString()).toStdString();
   // QString command = QString::fromStdString(m_helpFilePath);