Message with variables inside single-quotes are not displayed correctly. The variable names are displayed, rather than the values. Here is the list:

mobhunting.commands.hologram.created=The hologram '${hologramid}' was created
mobhunting.commands.hologram.deleted=The hologram '${hologramid}' was deleted.
mobhunting.commands.hologram.hologram_exists=The hologram '${hologramid}' already exists, please use another name!
mobhunting.commands.hologram.unknown=The hologram '${hologramid}' does not exist.

This one is probably similar:

mobhunting.bagofgoldsign.line3.not_a_number=Line 3 must be a positive number or '${everything}'

I just tried "/mh hologram delete test4" and it worked on my test server???

Spigot 1.12.1 latest
MH 5.0.9-b465
HolographicDisplays 2.2.6

I am getting different. For example:

The hologram ${hologramid} was updated.

Note that I removed the single-quotes from lang/en-US.lang

Nothing in log or in-game suggests any problem.

Strange, maybe its a linux/windows think. Im on linux, are you on windows?

I think i will solve this problem using colorcodes instead of ' '

CentOS Linux.

I have now colorcoded the hologramname instead of using ' '

Please try build 466
https://fractal.lindegaard.one:8181/job/MobHunting/