Fix 480937a103708a651/lyxgit. See also #9740.

Fix 480937a103708a651/lyxgit. See also #9740.

Actually, the changed tests were used to prevent overwriting the encoding
changed in Buffer::writeLaTeX with a language-default encoding.
This is still required for XeTeX with TeX-fonts unless a proper solution is found.

Documents with more than one encoding and TeX-fonts fail with LuaTeX,
as "luainputenc" can only handle one encoding.