Apparently, all Microsoft Word documents need at least one content language. This / These can be different from the user interface language. The content language(s) is / are used by Word for proofing, i.e. spell checking and grammar checking, if corresponding rules are available. Thus, choosing a different language (not Esperanto) for texts that are actually written in Esperanto, gratuitously marks many words as misspelled.
Is there a way to set the content language of a text in Microsoft word to Esperanto? (Regardless of whether spell checking or grammar checking won't be available and turned off.) Is there some way of adding custom content languages to the set of available choices that Microsoft didn't think of and/or doesn't provide a Microsoft Office language pack for? Or is there at least some way to let Word know that the content language for some text is "none of these" or "other" or something like that?
Note that this question isn't about the user interface language, which is a much less pressing issue, as most Esperanto writers are capable of reading at least one other language.