Use Buffer::validate() when previewing single paragraph

We used to use BufferParams::validate(), which does not validate the
paragraphs.

src/Buffer.cpp

@@ -3755,7 +3755,7 @@ auto_ptr<TexRow> Buffer::getSourceCode(odocstream & os, string const & format,
 			// in order to know if we should output polyglossia
 			// macros (instead of babel macros)
 			LaTeXFeatures features(*this, params(), runparams);
-			params().validate(features);
+			validate(features);
 			runparams.use_polyglossia = features.usePolyglossia();
 			texrow.reset(new TexRow());
 			texrow->newline();

status.22x

@@ -79,6 +79,9 @@ What's new
 - The '|' special character in index insets must no longer be inserted via
   TeX-mode.
 
+- Fix preview of language switching commands for single paragraphs in some
+  corner cases.
+
 
 * LYX2LYX