Judging by uniforms and equipment, they're using the western front (classic Germany vs. Britain and France setup), at least for the trailer.
The prominence of the West Front is mostly because the West Front trenchwarfare is the most iconic imagery from World War I - which is justified considering the trench warfare is part of the reason the staggering number of casualities on both sides, which are why this war remained so ingrained in European memory.
A WWI Battlefield game? Well, I suppose it's good to see that conflict get some recognition. I don't think they're going to go for a lot of realism here, though. WWI is hard to portray well in a game because it had clumsy weapons, huge, largely static battles (especially early on) and copious amounts of causalities.
Only on the West Front in Europe (where the mobile warfare of the early war came to an halt after the first year, due to poor planning and execution on both sides - and dragged on the iconic trench warfare until attrition and bad management on part of the Middle Powers, as well as the US reinforcements lead to the Entente victory) ... Warfare on the Eastern Front was more mobile but since Eastern Europe was less developed in terms of modern roads and railways that mobility does not transfer into huge movements.
And trailer also shows what seems to be combat against the Ottoman Empire, which I am glad is included just to break up the image that World War I was the West Front. The part of the other Middle Powers (Austria-Hungary, the Ottoman Empire and Italy) played is usually glossed other (unless of course in Australia and New Zealand, see ANZAC Day) despite it being highly significant for the state the Middle East is in today and the situation on the Balkan during the entire 20th century (as extension of the policies of 19th century).
EDIT: they might of course ditch realism to further gameplay, mind you... DICE is running on a "gameplay first"-philosophy after all
Something that is going to be interesting to observe is if and how they are going to deal with the topic of "colonial troops" in either army (the French had units recruited, often forcefully, from their colonies; the British Empire also conscripted troops from around the world; Germany leveraged/recieved help from China for their logicistcal support etc etc etc). The status and conditions of non-european troops in Europe were pretty abysmal for the most part....
What we will probably not see is the face of World War I in Africa ... because that is some heavy stuff european chauvinism at best and brutal racism at worst. I don't see how an AAA-game is going to dive into that because that's going to drive a very vocal internet crowd into frenzy and no one is going to want that PR job then. Even if DICE can currently leverage a great deal out of the EA at this point...
