Word Strength doesn't decrease after tree completion
At least I think it doesn't. I completed my Spanish Skilltree a few days ago and haven't refreshed skills a lot. Before all modules were finished every day a few of them would lose the full word strength status, now not a single one has gone down after a few days.
Hi! The word strength will decrease over time. If you finish the tree the idea is that you would go back to brush up on skills that are not as strong as they once were. You'll see them decay with a little more time. We are tweaking the algorithm behind this :)