When a document (or some characters) are marked as one language, while the
spellchecker's dictionaries do not include that language, the spellchecker runs,
but reports zero errors. 

This is "correct" operation (obviously don't want French checked against and
English dictionary), however as evidenced by numerous bug reports and forum
queries, it continues to annoy users.  What is missing is some feedback that
some words, perhaps all of them have not been checked because there is no
appropriate dictionary installed.

Also, who knows what language an imported document appears to be.
To the average user, the document is in "English" and they have an "English"
dictionary installed, so they expect it to work in some fashion.

For instance, as I am in New Zealand, it seems sensible to set the default
language to "English (New Zealand)".  However there is no specific NZ
dictionary, so the best I can do is install the Australian or GB dictionaries.

Spell check quite  happily reports "The spelling check is complete",  with no
errors reported.  I think it should also say "500 words not checked because no
English (New Zealand) dictionary is installed"

Help text for this message would include info about installing additional
dictionaries, and how to change the language of an existing document (select
all, format/character:language).

An additional feature would be the ability to tell the spellchecker to use a
particular dictionary for another 'language' (In my case I'd like some way to
say "Use English (Australia) dictionary for English (New Zealand) text"

BTW I'm calling this issue a DEFECT because I consider it a UI defect that the
checker gives the appearance of working when in fact it is ignoring some or all
of the text.