Hello !
I want to use the Landscape Mountain terrain in one of my game (created with Unreal Engine). I just want to use it to focus on the “technical part” of my game, and I’m going to replace it with my own terrain after. Is it authorized ?
Sorry for my bad English 
If you are talking about a marketplace Asset then yes, it is authorized. If you’ve bought it then you can use it in whatever project you want.
That’s a free asset from unreal engine’s creators. I assume that’s the same ?
If you thinking about the “Landscape Mountains” (under the “Learn” tab in the UE Launcher), than yes, the only restriction is for the pack from Epic is to use the materials in an UE project, so; yes. 
Ok, cool !