If Document>Settings>Language>Encoding is set to any value except "auto" or "default", we
expect the whole document to use this encoding. Wiht encodings from the CJK package, this means
one big "CJK" environment and no encoding switches.
Characters that are not handled by the CJK package need to be "forced" in lib/unicodesymbols.
This is completed for "euc-cn", the others will follow.
