That's perfectly normal.
Preparing to launch minecraft client for 1.12.2 - The launcher has been told to run Minecraft version 1.12.2.
Checking installations. - The launcher looks in your versions folder for 1.12.2. If it fails, it downloads it.
Minecraft client 1.12.2 is ready to start. - Version 1.12.2 exists in your versions folder and is not corrupt.
Starting! - The launcher is now actually launching the game.
Using default game log configuration client-1.12.xml (outputs XML) - These are just settings for the game to keep track of how you'd like the launcher to log everything in the console.