Snippet text wrote: I suspect he switched to that language when he wrote down things that might have done serious damage to the cause of Chihiro and Schueler if it had fallen into someone else’s hands. Or perhaps they were things he might not have been certain of in his own mind at the time he wrote them down. From a handful of entries in the part I could read, I think it was a combination of the two.
If he was Chihiro/Schueler supporter who began to have doubts, because of the course of events or new info, he might well want to avoid damaging the cause or 'corrupting' his friends among the general Safehold population in case he was wrong, hence the
Spanish which protected him and others from their casual exposure to his ideas. Much easier and more natural to write in a language you know and others don't than to encode text by hand. As time went on his disaffection obviously grew, hence more Spanish. By that time it was probably habitual to write the secret stuff in that language.
Lots of people write journals to help them think through their own lives and issues and we have no evidence whatever that he made any effort to have his journals preserved or expected others to do so. Remember RFC said Kohdy had nothing at all to do with organising the SSK.
It doesn't matter if Merlin reads Spanish, because OWL certainly can!