Fix multiple language substitution in DX_LINGUAS.

For some reason this macro inserts newlines between successive values
in the calculated POFILES/MOFILES settings.  This causes syntax errors
when these get substituted into makefiles and is very, very broken.

So this macro has basically never worked except for trivial cases.  This
happens when you don't bother to test things.  So let's fix that up and
add a test case to expose this issue.