Hi,
I'm using Uwe's binary (LyX 2.0.1 on Win7 64bits, Miktex 2.9), so
perhaps it's the cause of this inconsistency.
About the Document-> Settings-> Language, section B9 of the User's guide
states:
---------------
Default is the package babel.
Automatic selects the most suitable language package for the view/export
format you will use. For example for the XeTeX formats the package
polyglossia will be used instead of babel.
---------------
However, when I use Default or Automatic, generating the pdf with XeTeX
result in failures, with an error message mentionning polyglossia(*).
If I switch to "Always Babel", compilation works fine. So I have the
impression that either the documentation is wrong or that LyX does not
do as expected.
Best regards,
Olivier.
(*) It seems XeTeX and polyglossia have had issues in the past weeks,
but may have been fixed very recently. But that's besides the point.