English is spoken more in bigger cities and more among young people. German is widespread, too, more spoken by elder ones. In all castles, it should be possible to get some information in English and German. The train is a good access possibility if you want to take the bike with you as well as if you want to travel within the country. In my opinion, it's better for you that you travel along with it, for sake of the way your bike might be treated otherwise.
Deviations are always possible.
The top tourist season is in July, less in August. Nevertheless, the main stream of tourist heads to Prague, so elsewhere you'll feel good off more or less any time. If you decide to stay on camping sites, you won't have much troubles with finding them, especially in July- August. Others Bicycle stealing doesn't happen very often, so your bike should be safe with a normal lock if you leave it for some time. If you want to feel more secure, it is never a bad idea to ask somebody to watch it, if you have this possibility.