Problem with multilingual switched off

"Edit As HTML" disappears after switching off Multilingual in
XWiki Preferences.

And another, I suppose, similar problem:

I wasn't able to switch multilingual off because of a strange
behaviour:

• set in xwiki.preferences language and default language to pl,
  multilingual to off
• edit Main.WebHome, save
• edit Main.WebHome again - you have still the original (en) content

Appending '?language=pl' to edit url doesn't help.

The same happens with left menu - xwiki cannot displan non-en version when multilingual is switched off.