diff --git a/src/Buffer.cpp b/src/Buffer.cpp
index 2f06f18e37..4d3088458f 100644
--- a/src/Buffer.cpp
+++ b/src/Buffer.cpp
@@ -1876,6 +1876,47 @@ 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.
diff --git a/src/LaTeXFeatures.cpp b/src/LaTeXFeatures.cpp
index db2aa74cb7..96f0ced7e8 100644
--- a/src/LaTeXFeatures.cpp
+++ b/src/LaTeXFeatures.cpp
@@ -683,6 +683,42 @@ 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;
diff --git a/src/LaTeXFeatures.h b/src/LaTeXFeatures.h
index 38273e4fa2..6c3e888b48 100644
--- a/src/LaTeXFeatures.h
+++ b/src/LaTeXFeatures.h
@@ -122,6 +122,10 @@ 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;
 	///
diff --git a/src/Language.cpp b/src/Language.cpp
index dc38bfcf24..760d7a1ba7 100644
--- a/src/Language.cpp
+++ b/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())
diff --git a/src/Language.h b/src/Language.h
index ed1bd8715e..55f4b76f21 100644
--- a/src/Language.h
+++ b/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)
diff --git a/status.22x b/status.22x
index 0a10e6ae28..29a63088bb 100644
--- a/status.22x
+++ b/status.22x
@@ -47,6 +47,9 @@ What's new
 - When using Return in an empty paragraph and the depth is reduced,
   the layout of the paragraph is now changed accordingly.
 
+- Issue an error message if incompatible languages (in terms of language package
+  requirements) are used (bug 10456).
+
 
 * DOCUMENTATION AND LOCALIZATION