About using Reward Miles...
You are not considered second class versus paying customers. In fact, a reward mile customer shows their allegiance to that airline while that other "paying" passenger may be flying on the cheapest ticket possible and not be a FF. If you book on one airline and are placed on another, the same thing is happening in reverse on those two airlines.
And don't think the partner airline will treat you badly. I was having a serious problem with a reward flight booked on American but scheduled on British Airways. AA didn't want to help unless I paid $200 and then wouldn't keep me on the same flight. BA stepped up to the plate and after a couple of emails, fixed the problem with lots of apologies that it took so long. Not one bit of caring from AA.
If you earn miles any way besides those given by the airline, then those miles were purchased by that third party. Your credit card gives miles, the bank pays for those miles. You shop using the shopping portal offering miles, the airline makes money. So, those miles are not free from the airline. They've already made money on them. (Making it difficult to book flights just shows how greedy the airlines can be. They've made money on miles and now don't want you to use them so they can make more selling seats rather than letting you use them.)
As for English, I don't think you'll have a problem at the airport. English is the most common second language and every airline will have staff members who speak it.