Fix #10778 (issue with CJK and language nesting)

When using CJK, don't try to close a language that was never
opened before, such as when it is the main language.

(cherry picked from commit 7e51b5f301)

src/Paragraph.cpp

@@ -2542,6 +2542,7 @@ void Paragraph::latex(BufferParams const & bparams,
 		bool const using_begin_end = runparams.use_polyglossia ||
 						!lang_end_command.empty();
 		if (!running_lang.empty() &&
+		    (!using_begin_end || running_lang == openLanguageName()) &&
 		    current_font.language()->encoding()->package() == Encoding::CJK) {
 				string end_tag = subst(lang_end_command,
 							"$$lang",

src/output_latex.cpp

@@ -532,6 +532,14 @@ void popLanguageName()
 }
 
 
+string const & openLanguageName()
+{
+	OutputState * state = getOutputState();
+
+	return openLanguageName(state);
+}
+
+
 namespace {
 
 void addArgInsets(Paragraph const & par, string const & prefix,

src/output_latex.h

@@ -39,6 +39,13 @@ void pushLanguageName(std::string const & lang, bool localswitch = false);
  */
 void popLanguageName();
 
+/** Return a reference to the last active language opened with
+    polyglossia or when using begin/end commands. If none or when
+    using babel with only a begin command, return a reference to
+    an empty string.
+ */
+std::string const & openLanguageName();
+
 /** Export optional and required arguments of the paragraph \p par.
     Non-existing required arguments are output empty: {}.
  */