Steffen: can you please be more specific?
As I read the seciton 8.2 of the cited document, I do not see a disparity with my statement. There is even an example:

"""
For example, if a user wanted to interact with the system in French, but required to sort German text files, LANG and LC_COLLATE could be defined as:

    LANG=Fr_FR
    LC_COLLATE=De_DE
"""

which is (almost) exactly my case. I have LANG set to en_US to tell the system that I want to interact in English, and LC_CTYPE - to Russian to tell it that "classification of characters" needs to be Russian-specific.

Note that I do *not* have LC_ALL set, because it takes precedence over all other LC_* variables which is *not* what I want.

I believe that the correct "guessing order", according to the document that you cited, would be:
  LANG
  LC_ALL
then possibly (possibly because it does not have encoding info)
  LANGUAGE
then optionally, as a last resort
  LC_CTYPE and other LC_* variables.