The bug

There is no translation for "Air" when used in the /replaceitem command. Using it in other commands like /give @p air will translate it correctly.

How to reproduce

1. Set the language to a non-English one
2. Use the following command

   /replaceitem entity @p slot.armor.head minecraft:air
   

Code analysis

Based on 1.11.2 decompiled using MCP 9.35 rc1

The method net.minecraft.command.CommandReplaceItem.execute(MinecraftServer, ICommandSender, String[]) uses the String "Air" if an item is empty. This method also tests after it could not get an item for the id if the id matches the air block id.
Both special cases can probably be removed because some versions ago air became a valid item that's instantly removed.