Hi Jim:
You are not going to get a more sensible answer than Phillip's
You need
to read up the whole subject of "Language" in the Word Help. There are
several topics, read them all, to understand how this works.
What Phillip is saying is that Word senses which keyboard your computer has
in use when Word starts. It sets the first language appropriate for that
keyboard as its "current" or "working" language.
When Word encounters text in the document, it then checks to see if a
language has been marked. The language marking can be in the text, the
sentence, the paragraph, the style, the parent style, or the Normal
template. It looks down the list in that order until it finds a language
setting. If it does not find a language setting, it continues to use the
one set by the keyboard.
There: That will save you a bit of reading.
But it's no more sensible than Phillip's answer, now is it?
Cheers