Note: I tried adding humor to this tutorial, so people find it easier :smile.gif:
I know what you're thinking...
OMG I NEED TO GET A SPECIAL KIND OF PROGRAM TO OPEN THE APPROPIATE FILE!!!111one
Truth: You don't even need Microsoft Word...
Step 1: Get 7-Zip File Manager.
Step 2: Go to AppData\Roaming\.minecraft\bin
Step 3: Open minecraft.jar using 7-Zip (right click, then Open With)
Step 4: Open the lang folder. See that en_US.lang thingymahoozit? Open that up with (insert text editing program here)
You will a bunch of text... only edit the text past the =. This way you can edit Multiplayer "failed to connect" messages, tooltips, and even game difficulties. Don't bother with the .desc, that doesn't work.
Step 5: Have some fun editing.
Step 6: Save your work. Ensure Minecraft is closed before you do so.
Step 7: Let 7-Zip update the .lang. If it doesn't work go up one folder and delete the META-INF folder.