I had the following console message on upgrade:

MU Control registered upgrade mapping Battery Powered Locomotive to MU Unknown key: "entity-name.BatteryPack-locomotive"

the locomotive BatteryPack-locomotive has an explicit localised name, as it is derived from locomotive's localised name.

You possibly want to either check to see if a localised_name is defined, or you could perhaps use a library like
Rusty's Locale Utilities to take care of thinking about that for you.

I realized I was not doing the simplest way to handle this, which is to check if <entity>.localised_name exists before overwriting it. This should be fixed in the latest version now.