What is the Standard dictionary used for? How does it differ from language-specific dictionaries?
If you are referring to the dictionary that you find via Tools | Language Settings | Writing Aids in the User-defined dictionaries section; that is the default location for adding user defined words that are not in the main dictionary. You can also create other dictionaries - perhaps you might want a specific dictionary for medical, building or other terms that don’t appear in the main dictionary.
Edit 2018-02-25
LO 5.3.7.2 on Windows 7. A default install has custom US and UK English dictionaries with some default content but it doesn’t have a standard dictionary, unlike AOO; you can choose to edit these and add/remove as needed. These are not the same as the main dictionary of whatever language you use.
When you create a new, or edit an existing custom dictionary (not the default ones supplied), one of the choices you can make is whether the dictionary applies to one specific, or all, languages.
You might try experimenting a little - create and activate a new dictionary, run a spell check on a document with known errors and see what options are available to you for correcting and adding new words.
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Thank you. I read that that is what the Standard dictionary is, but the name makes no sense. It seems it would be far more appropriate to call it the user-defined dictionary.
Anyway, how does it interact with the other dictionaries? That is, is there only one Standard dictionary, regardless of language?
It looks like a number of words have been manually added to the English-US dictionary. How is it determined that those words are in that dictionary rather than the Standard dictionary?
Thank you again for the additional answers. I have never found much of an explanation of how the dictionaries work and appreciate the time you took to clarify.