Surely a big worry for Western countries if temperature is going to 42 degree Celsius then surely they are going to suffer and have side effects of this as well because Qatar is almost desert, and they are not having any better environment for these Western players those are mostly playing into cool weathers. This is going to be beneficial for South American countries as their players are well-prepared for this all, and they can't suffer like European players and this advantage can give them their first success after two decades, and they can win this year's event.
Qatar's authorities and FIFA officials are already working on few things which can used for first time because it's first world cup into the Middle East, so they want to do their best for having their desired results but still it's not easy to play and giving best results in these conditions.
They will mostly face this issue of high temperature/ hot weather conditions within their few weeks (1week-2weekd) of their exposure /presence to high temperature in Quata, So I do think that adaptation/acclimatization to that environment will start to take place, take for instance people who comes from extremely hot weather environment to play football in Europe ,what happened when they gets to cold weather environment, you will realize that at a particular they got adapted.
Too I think some measures will be in place , maybe FIFA would likely allow up to 5 or more substitution to take place and equally have a brief period where players will have theirselves cooled down a bit.