Routing doesn't really work offline, but that's a different/harder problem.
Google will probably work ok for the most popular trails, and I guess you can use it as a supercharged compass. But at least in Europe, if you actually plan a route in any mountains based on Google, you're in for an adventure :)
Bit focused on eastern Europe, but has a hiking mode with very good coverage of the official routes (not only in eastern Europe ;)). And everything is free, including offline maps. Terrific value for casual trips.
https://play.google.com/store/apps/details?id=com.bergfex.to...
This one is not free (there is a free tier though) but seems to have more details in some areas, so the pro tier may be worth it if you hike a lot.