I have recently (and at long last) aquired the Breitkopf 2002 score of Luonnotar, with a preface written in 2005 by Timo Virtanen.

In the footnotes to the preface, there are excerpts from Sibelius' diary, written in Swedish and then translated to German/English.

The question: Did Sibelius really keep a diary in Swedish?

I have seen the published diary. I couldn't read much of it. It's in Swedish.

The bulk of the entries come from the period when he was most active as a composer. They kind of peter out as you get to the 1920s. So it seems they don't offer any clue as to what made him stop composing.

Yes, in Swedish... his first language, of course.
There is the occasional word in other languages too, but I don't remember any full entries in Finnish (I'm travelling now so can't check!).

Sibelius's diaries reveal his anger at poor reviews, but surprisingly on one page they show his disappointment at being unable to find one aristocrat among his ancestors. Sibelius was particuarly irate when genealogists traced his ancestry to Finnish peasants.
Still, I think it was the French musicologist Mark Vignal who described Sibelius has 'The aristocrat of Symphonists'--kullervopete.

Yes, quite so – and in 1919, shortly after that article appeared (it was in the newspaper Helsingin Sanomat, by the genealogist Eeli Granit-Ilmoniemi) Sibelius almost had a fight with [the writer] Eino Leino at a well-lubricated Kalevala Society gathering. Leino had been unwise enough to venture the opinion that Sibelius was 'a rococo figure and descended from Häme peasant stock'.