I was interested in possibly adding some Polish channels, however I wanted to preview them before I commit, even for a month. There is no way to do this?
No. What you could do is subscribe to the language package with the Polish channels you want and watch it for whatever time you like. If you like it keep it, if not, get rid of it. Everything will be pro rated so you'll only be charged for how long you were subscribed to it.
As long as its not an actual channel tier (prime, extreme, ultimate) and is just a premium channel add on you can subscribe and unsubscribe anytime you want without effecting or re-newing your contract if you have one and again will be pro rated to however long you had it.