Is it possible to display item name on sign in client language? Most people understand english, but for my younger sister and her friends it's a small issue. For example plugin Quests does it. There was small issue, but dev fixed it. Maybe you can use it as reference PikaMug/Quests@ebe13ea

That's not easily doable as the sign name in /iteminfo as well as on the sign itself are used to parse which item a shop sells. Changing this would only really be possible globally, not per-client (unless tons of work is put into doing some way to support per-client IDs on signs) and would also require that someone provided server-side translations for all items in the game in all supported languages which feels a bit over the top.

This would be a lot easier if we could just use translatable components that use client-side translations of the item names on signs (like the item names in the transaction messages when using ShowItem) but unfortunately not all names fit on the sign and it would obviously break if some player copied such a sign by manually typing it onto another one.

I will leave this issue open though in the case that someone comes up with a better idea or the underlying design of the plugin changes and makes such kind of things easier at some point.