us vs en? dealing with spells_us and eqstr_us

[smuggler]

Both my spell and my string txts have two files, spells_en.txt/spells_us.txt and eqstr_en.txt/eqstr_us.txt

When I edit the spells or strings with Mongrel's Spell editor, only the _us file is actually edited.. the other stays exactly as it was.

So... is the _en file just a backup or something? can i just copy the edited _us file over onto the _en file. Will that do something horrible that I don't want to happen?

I'm only asking because though they appear to be identical, the _en is actually a handful of Kb smaller than the _us... leading me to believe that it serves some other purpose. But hey, I donno, thats why I'm asking, heh.