Linux: fix language loading when running as AppImage

Fixes #1624

The language file path was hardcoded to /usr/share/veracrypt/languages/
which doesn't exist inside an AppImage runtime. Language files are
actually located under $APPDIR/usr/share/veracrypt/languages/ when
running from an AppImage.

This affected both the language file loading in Resources.cpp and the
language enumeration in PreferencesDialog.cpp, causing the language
selection to show only "System default" and "English" regardless of
which translations were packaged in the AppImage.

src/Main/Forms/PreferencesDialog.cpp

@@ -22,6 +22,9 @@
 #include "Main/Main.h"
 #include "Main/Application.h"
 #include "Main/GraphicUserInterface.h"
+#ifdef TC_LINUX
+#include "Platform/Unix/Process.h"
+#endif
 #include "Volume/Cipher.h"
 #include "PreferencesDialog.h"
 
@@ -132,7 +135,16 @@ namespace VeraCrypt
 #if defined (TC_MACOSX)
 		wxDir languagesFolder(StringConverter::ToSingle (Application::GetExecutableDirectory()) + "/../Resources/languages/");
 #else
-		wxDir languagesFolder("/usr/share/veracrypt/languages/");
+		wxString languagesFolderPath("/usr/share/veracrypt/languages/");
+#ifdef TC_LINUX
+		if (Process::IsRunningUnderAppImage (StringConverter::ToSingle (wstring (Application::GetExecutablePath()))))
+		{
+			const char* appDirEnv = getenv ("APPDIR");
+			if (appDirEnv)
+				languagesFolderPath = wxString::FromUTF8 (appDirEnv) + "/usr/share/veracrypt/languages/";
+		}
+#endif
+		wxDir languagesFolder(languagesFolderPath);
 #endif
 		wxArrayString langArray;
 		LanguageListBox->Append("System default");