I think solipsist's point isn't that they have an excuse but that they have a reason -- being monoglot hurts them less than it would if they were e.g. on the European continent, so monoglossy (or whatever the right word is) isn't necessarily silly for them.
[EDITED to add:] Disappointingly, OED suggests that the right word is just "monoglottism".
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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