The following lines spell the word color wrong. In the US its spelled color while in the UK its spelled colour.

I recommend duplicating the file and name it en_GB.lang for the UK users and just fix the spelling error on the US language file.

msg.draconiumChestEditmode.txt=Colour edit mode =
msg.draconiumChestEditL1.txt=Right click with red green or blue dye to adjust the colour
msg.draconiumChestEditL4.txt=Right click with paper to copy the colour to other chests

Actually, that is a correct spelling, even for US english, it is simply a matter of preference, like center or centre. While it is the preferred spelling, it is not grammatically incorrect.

Alternatively you could move to Australia. Then it will be spelled "correctly"

It may not be grammatically correct to spell it color in either country, Mojang themselves also change the spelling depending on the language selected, so that should be taken into consideration to follow the games standards.