Issue an error message if conflicting languages are used

Some languages are only supported by Babel, some only by Polyglossia.

If these are combined, we issue an error message now.

Fixes: #10456

src/Buffer.cpp

@@ -1874,6 +1874,47 @@ void Buffer::writeLaTeXSource(otexstream & os,
 		// Write the preamble
 		runparams.use_babel = params().writeLaTeX(os, features,
 							  d->filename.onlyPath());
+		
+		if (!runparams.dryrun && features.hasPolyglossiaExclusiveLanguages()
+		    && !features.hasOnlyPolyglossiaLanguages()) {
+			docstring blangs;
+			docstring plangs;
+			vector<string> bll = features.getBabelExclusiveLanguages();
+			vector<string> pll = features.getPolyglossiaExclusiveLanguages();
+			if (!bll.empty()) {
+				docstring langs;
+				for (vector<string>::const_iterator it = bll.begin(); it != bll.end(); ++it) {
+					if (!langs.empty())
+						langs += ", ";
+					langs += _(*it);
+				}
+				blangs = bll.size() > 1 ?
+					    support::bformat(_("The languages %1$s are only supported by Babel."), langs)
+					  : support::bformat(_("The language %1$s is only supported by Babel."), langs);
+			}
+			if (!pll.empty()) {
+				docstring langs;
+				for (vector<string>::const_iterator it = pll.begin(); it != pll.end(); ++it) {
+					if (!langs.empty())
+						langs += ", ";
+					langs += _(*it);
+				}
+				plangs = pll.size() > 1 ?
+					    support::bformat(_("The languages %1$s are only supported by Polyglossia."), langs)
+					  : support::bformat(_("The language %1$s is only supported by Polyglossia."), langs);
+				if (!blangs.empty())
+					plangs += "\n"; 
+			}
+
+			frontend::Alert::warning(
+				_("Incompatible Languages!"),
+				support::bformat(
+				  _("You cannot use the following languages "
+				    "together in one LaTeX document because "
+				    "they require conflicting language packages:\n"
+				    "%1$s%2$s"),
+				  plangs, blangs));
+		}
 
 		// Japanese might be required only in some children of a document,
 		// but once required, we must keep use_japanese true.

src/LaTeXFeatures.cpp

@@ -736,6 +736,42 @@ bool LaTeXFeatures::hasPolyglossiaExclusiveLanguages() const
 }
 
 
+vector<string> LaTeXFeatures::getPolyglossiaExclusiveLanguages() const
+{
+	vector<string> result;
+	// first the main language
+	if (params_.language->isPolyglossiaExclusive())
+		result.push_back(params_.language->display());
+	// now the secondary languages
+	LanguageList::const_iterator const begin = UsedLanguages_.begin();
+	for (LanguageList::const_iterator cit = begin;
+	     cit != UsedLanguages_.end();
+	     ++cit) {
+		if ((*cit)->isPolyglossiaExclusive())
+			result.push_back((*cit)->display());
+	}
+	return result;
+}
+
+
+vector<string> LaTeXFeatures::getBabelExclusiveLanguages() const
+{
+	vector<string> result;
+	// first the main language
+	if (params_.language->isBabelExclusive())
+		result.push_back(params_.language->display());
+	// now the secondary languages
+	LanguageList::const_iterator const begin = UsedLanguages_.begin();
+	for (LanguageList::const_iterator cit = begin;
+	     cit != UsedLanguages_.end();
+	     ++cit) {
+		if ((*cit)->isBabelExclusive())
+			result.push_back((*cit)->display());
+	}
+	return result;
+}
+
+
 string LaTeXFeatures::getBabelLanguages() const
 {
 	ostringstream languages;

src/LaTeXFeatures.h

@@ -124,6 +124,10 @@ public:
 	bool hasOnlyPolyglossiaLanguages() const;
 	/// check if a language is supported only by polyglossia
 	bool hasPolyglossiaExclusiveLanguages() const;
+	/// A vector of all used languages supported only by polyglossia
+	std::vector<std::string> getPolyglossiaExclusiveLanguages() const;
+	/// A vector of all used languages supported only by babel
+	std::vector<std::string> getBabelExclusiveLanguages() const;
 	///
 	std::string getBabelLanguages() const;
 	///

src/Language.cpp

@@ -44,6 +44,12 @@ bool Language::isPolyglossiaExclusive() const
 }
 
 
+bool Language::isBabelExclusive() const
+{
+	return !babel().empty() && polyglossia().empty() && requires().empty();
+}
+
+
 docstring const Language::translateLayout(string const & m) const
 {
 	if (m.empty())

src/Language.h

@@ -44,6 +44,8 @@ public:
 	std::string const polyglossiaOpts() const { return polyglossia_opts_; }
 	/// Is this language only supported by polyglossia?
 	bool isPolyglossiaExclusive() const;
+	/// Is this language only supported by babel?
+	bool isBabelExclusive() const;
 	/// quotation marks style
 	std::string const quoteStyle() const { return quote_style_; }
 	/// requirement (package, function)