I have a few suggestions for people who take public transport and also try to learn a new language. However, here is only one of them for you today.
This will work only if you are at a place where the target language is widely spoken. I had experienced this personally and it had definitely worked for me. Here is what you do:
Hop on a train/tram/bus and before taking a seat, eye out the people in it. If there is someone sitting alone and you are a chatty person, go sit next to him/her and start a conversation even if you are not very accurate or fluent in that language. If you see that there is a group of people (at least two people) talking to each other, try to sit somewhere close by to them and listen. I know it doesn't morally sound good but hey, you are only trying to do this as a part of your language studies, not to really listen to what they are talking or gossiping about. Especially if you are a shy person or don't have enough confidence to talk to people at this stage, this may work better for you. You can't necessarily practice your speaking but you can improve your listening skills by doing that.
Another point I would like to make even if it may not sound right is that try to sit beside/around people that look like native speakers of your target language. For instance, if you are in Japan, trying to improve your Japanese, then try to sit close to 'Japanese looking' people rather than Middle Eastern or Western looking ones. You can always be surprised at who speaks a language better than who but if you need to take a guess, then this is the safest.
I also highly recommend you to take notes especially if you sit on the seat behind the people that you are trying to listen to without making it too obvious. If you hear a word that you are unfamiliar with, just try to remember it and jot it down as soon as you can. You may not be able to write it correctly but then you can ask for help from a friend/teacher etc later on for correction. Additionally, pay attention to the way they pronounce the words that you are familiar with as well. You may see that they may be pronounced differently than you think they do.
I need to remind you once again that the intentions here are not to interfere with anyone's privacy or disturb anyone. Nor is it to discriminate. The idea here is that you use the time you are on public transport as an opportunity to practice your language skills by trying to catch some words/sayings/idioms/cultural aspects that are associated with that language that native speakers of that language have, pronunciation, intonation and so forth.
Have a day when you learn something useful today!