Signs you should learn the host country language:
1. You are studying a degree that is NOT offered in English. I believe this can go unsaid. Nevertheless, if you were even considering a degree in a different language, I doubt you would do so without already having some language skill.
2. Do study abroad programs have a minimum language requirement? Even if you are not a student, this can be an indicator. Logically, a study abroad program would not want to turn students away ($$$), so a minimum language requirement indicates that some skills is necessary for functional living.
3. Do universities even offer degree programs in English? If they don't offer any or only offer very few, its best to assume you should have a basic grasp of the language.
4. Do government/university web sites have the option to read their content in English? If not, it could be assumed that accommodation in English is not widely available.
Even if you conclude that you do not need to learn the host country's language, its still advisable to learn a few survival phrases. For example, in the case of an emergency situation, you may have to phone for help or communicate with a medical professional. Overall, making an effort to communicate in the foreign language, even if its just "Hello," "Good day," or "Do you speak English?", will most likely make the locals more receptive towards you.
I'm a bit nervous that I will be going to Finland with very little language skill. They offer many degrees in English, and my internship does not require Finnish language skills. However, I'm a lover of language, and I still want to learn. Unfortunately, in a global scope, Finnish has few speakers and little demand for Finnish language software. I was disappointed to find Rosetta Stone did not support it. Luckily, I was able to find a simpler software on Amazon.com this summer and will be traveling with a limited vocabulary of "yes," "no," "thank you," "you're welcome," and "good day." Not a whole lot to go on, but its a start.