Per @SalmonSnake's suggestions. Took a bit longer than expected, since I encountered a few more bugs that need fixing.

• Messages are now less likely to show double
• Clone spawn rate now matches what they are configured to (was far higher)
• Clones no longer drop items when killed naturally
• Added hunger drain option when going far enough. Off by default.
• Added regeneration disabling option when going far enough. Off by default.
• Added option to make special last resort options ignore experience level; or pay attention to it when mobs ignore it. By default, both pay attention to experience level.
  
• Added a maximum relative level. This applies to mobs and last resort options so be careful not to set it too low. Off by default.

Added custom language support. Be sure to rename default.yml, or it will be overwritten. Set it with ` locale` in the config.
Also added the option to use a square world border. This is now ` enabled` by default, and to disable it, turn on ` smooth-distance`. This should provide some performance gains.

• Fixed spawning quadruplets almost every time when the relative level was low enough
  
• Changed alias to /buff from //buff

Mobs will now become faster and see you from further away as you level up. These were avoided because they permanently affect the mobs, and can create some unfairly fast monsters. The provided defaults seem fair enough.

If players go way too far for their level, three special effects have been added:

• Forced aggression: Neutral and hostile mobs near you will target you, even if invisible or in creative mode. This makes the End and the Nether quite dangerous. Defaults at level 20 (about 3200 blocks out by default).
• Passive damage: Simply take damage over time. This damage ramps up the farther you go. Defaults at level 40 (about 6400 blocks out by default.)
• Smite: Immediately die at a certain distance. This is not enabled by default, since using a vanilla world border is probably preferable.

More configurations have been added to accommodate these new features.

If you update from an older version of this plugin, you may need to reload the configuration.

The plugin no longer panics when you're using versions below 1.11.

Note to server owners:
You must set the ' use-subtitles' flag in the configuration file to false! There are safety nets in place if you are using versions 1.8, 1.9 or 1.10, but anything below that will not tell the player when they are in danger and instead spam your console with errors.

Changed some text to be more descriptive and accurate for the config.yml.

In order of importance:

• Damage is now registered correctly! All (most) sources of player damage are now counted against you while fighting, not just melee. Bows, potions, TNT, and wolves are all confirmed to work.
• The config now loads correctly and has new features. It technically had one already, but it didn't load correctly. Everything you can change is now fully documented in the file and on the main page.
  
• The display has been moved to the game's subtitles instead of chat, so it will be a little less spammy and a lot more noticeable.
  
• Multiworld now works consistently and how you want it to.
• It's faster for servers with players who aren't AFK.

Basically everything works now, except for a few compatibility issues. The only confirmed one is that mcmmo unarmed damage is absolutely broken and will deal more or less normal damage.

Mob incoming damage would not be affected, and PvP was broken due to an oversight.
Will do more extensive testing before releasing next time! This is a small plugin and more testing was done, so there shouldn't be too much more that can break.