I found a few bugs.

1. Conflict with Thaumcraft 3 mod.
   If you connect 2 mirrors (I hope you know how they work) and aim at one of them, the game console spams an error of the following type:

[SEVERE] [Minecraft-Client] ########## GL ERROR ##########
[SEVERE] [Minecraft-Client] @ Post render
[SEVERE] [Minecraft-Client] 1283: Stack overflow

This blows up the .log file very quickly. But it doesn't seem to affect the performance of the game.

2. Conflict with InGame Info XML mod. Visual flickering bug.
   If you change the value in the BlockHelper.cfg file to something like:

    # Size factor for the tooltip
    D:Size=0.7

and aim at the mirror from TC3 again, then the font starts flickering. And some other interface elements from other mods. For example, HUD from Ars Magica.
It's just a combo :D After a game session you get a huge .log file.

Quick info about mirrors. Just in case.
https://www.youtube.com/watch?v=Qcq2kvknLdE

Definetly reproducible on my end, thanks for reporting these!
Actually, I don't even need to connect two mirrors. Just looking at any mirror already causes this behaviour.
I think I may know what's causing the issue, but I will try to investigate after I have successfully fixed the translator on every version :)