I accidentally added an unintended curly bracket to a /scoreboard players tag command, and in the output it said 'commands.scoreboard.players.tag.tagError'
To check if it does have its translation file, i set up a tellraw command that would translate the output, but it did nothing.
Steps to reproduce:
1. Get yourself a command block

/give @p minecraft:command_block 1 0

2. Place it in your world
3. Put in this command:

/scoreboard players tag @e[type=Item] add (any tag name) {Item:{id:"minecraft:blaze_rod"}}}

The additional curly bracket in the end is for triggering the translation file, the tags can be other than

and can be triggered on all entities.
4. Set the commandblock to Repeat mode, and to Always active, and if the button in the output is set to X change it to be O.
5. Exit the menu.
6. Drop a blaze rod.
7. Open the commandblock's menu once again.
8. Copy the translation in the output, (Ctrl+C/Strg+C/Cmd+C) but not the time.
9. Place an another commandblock, and open its menu.
10. Put in that command:

/tellraw @p {"translate":"(Paste the output, Ctrl+V/Strg+V/Cmd+V)","color:"red","bold":"true"}

and set it to be Always active
11. Exit the menu
12. The commandblock will say commands.scoreboard.players.tag.tagError
13. Delete the command, and type in this instead:

/tellraw @p {"translate":"subtitles.entity.generic.explode","color:"red","bold":"true"}

(those are the entries for the Explosion subtitle) Also, set it to Need redstone
14. Power the commandblock.
15. It will now say Explosion wich means, that the commands.scoreboard.players.tag.tagError output doesn't have the translation.
I have put the 'how i did that' in in a YouTube video, as the attachment is too big. https://youtu.be/_wcH7rVGLzU