The Tatra Mountains are a mountain range which forms a natural border between Poland and Slovakia, and are the highest mountain range in the Carpathian Mountains.
In both countries, the English translation of their part is "Tatra National Park". However, in Polish, it's Tatrzański Park Narodowy - while the Slovakian side the name is Tatranský Národný Park.
Tatra mountains span across border between Poland and Slovakia, so most people you may meet there will speak in languages of these countries, but communicative English is also very common. Staff at mountain refuges should be able to communicate in English too.
Most interesting part of these mountains is under national park protection, so if you want to see it, you will have to walk a lot. Mountain range has many trails for both experienced and novice hikers, so navigation is not a problem, but checking difficulty of trail before setting of is mandatory if you want to avoid unpleasant surprises. Another thing worth noticing is that some harder trails are one-way only, and can be pretty dangerous under bad weather conditions.
On the Polish side, the bus is the best way to get around outside the national park, because buses are cheap (3zł ~ €0.7), arrive and depart very often, and sometimes it is possible to negotiate the route.
There is a railway parallel to mountain range in Slovakia, so in addition to being convenient means of transport it provides astonishing views on High Tatras. In the winter it is also good for getting from one ski resort to another.