If the tour is guided in the Overview section on the tour page you can see available tour languages. Usually it means along the tour you have a professional tour guide guiding the tour in one or a few specified languages.
Some tours or Activities doesn't require the guide and have a pilots, instructors or simply provider's team member who take care of you along the tour. In this case it's marked as "Not Guided" tour, because it doesn't require a professional tour guide.
If an activity is conducted in your language, that doesn’t mean it is only conducted in your language. Many guides are multilingual and give tours in multiple languages. That apply especially to the cheap coach tours and big group tours. But don’t worry! You won’t miss out on any interesting and important information.