Don't use \n sequence inside Lisp strings needing translation...

... Because xgettext will just remove the \, not replace \n with newline.

That's consistent with Lisp reader behavior in this documentation:
http://www.lispworks.com/documentation/lw70/CLHS/Body/02_de.htm

The XLisp reader, which replaces \n with newline, is nonstandard.

So, to accommodate xgettext, use (format nil "...~%...") instead, or where
you can't do that in a $ header line, just make a line break inside the ""

There are a few "\n" left alone in sample-data-export.ny which are neither
in $ lines nor inside (_ "...")

plug-ins/crossfadetracks.ny

@@ -72,5 +72,5 @@ audio clip, fade in, otherwise fade out."
     (if (< in-dist out-dist) 'in 'out)))
 
 (if (< (length (get '*selection* 'tracks)) 2)
-    (_"Error.\nSelect 2 (or more) tracks to crossfade.")
+    (format nil (_"Error.~%Select 2 (or more) tracks to crossfade."))
     (crossfade type direction curve))