I did a number of skills in the Japanese course months ago, then let it lapse and didn't touch it. Five days ago, I went through all the decayed skills and strengthened them until they were golden.
Since then, I have been completing one skill per day. However, since my strengthening session five days ago, I haven't had a single skill decay on me, neither from the old ones nor the ones I've done now for the first time. This doesn't feel like the courses usually behave.