While working, my Indesign changes the dictionary in my default settings from German: 2006 spelling reform to 1996.It happens in a wide variety of situations.When inserting text (in a wide variety of ways) when I click a text field.I don't even have to have a document open.If I open the presets more than once and don't exit with OK but with Cancel, then it also changes.
Spell check languages are called from within Paragraph and Character styles. Have you examined your paragraph styles? What dictionary language is active with no documents open and you edit "Basic Paragraph Style"? I recommend you also make sure that there is no character style being constantly applied simply because Character Style panel isn't on None but accidentally clicked on some other character style (that might also be applying the wrong spell checker language.)