Ugly fix for bug #9102

Ugly fix for bug #9102

The real problem is the encoding of latex_language: It is hardcoded to latin1,
but InsetListig uses the currently active encoding. Therefore, we cannot tell
whether any given character wil be encodable or not, and we should not prevent
non-ACII characters.
In the future, we need to make the encoding of latex_language dynamic, so that
it always represents the currently active encoding. Then, we could do the
correct check both for listings and ERT. For now, I simply disabled the
encoding check for listings, which also means that bug 9012 might occur in
other cases for listings, but this is less important than bug 9102.