The Scribunto debugging console (when you edit module, under the edit window) runs in English instead of the site language.
So when you have (trivial artificial example) module "January" on cs wiki containing eg.:
local module = { plain = mw.message.new( "january" ):plain() } return module
then calling
j = require( "Module:January" ) -- some other code print( j.plain )
will print leden while calling
print( p.plain )
in debug console will print January