The "Custom Player Name" and "Custom Resource Pack" skin options on an npc trainer don't seem to like British English.
When it's set as the language, those two options make the npc use a missing texture. Additionally, the textbox you would normally type in with those two options disappears.
This issue is fixable by switching back to American English and restarting the game.
The log was a key component in figuring out what was wrong as it has a load of NullPointerExceptions with the gymleader files, just before an error stating that it could not load the textures needed to use the 2 skin options.

https://pastebin.com/MLCg9Uep
This is a complete client log file, starting the game, logging into a singleplayer world, testing the npc skin and closing the game.

Thanks for the report. I was able to confirm this, and have passed it along to internal. Keep an eye on the ticket status to see what's up.

Fixed in upcoming