class I18n::Config

def missing_interpolation_argument_handler=(exception_handler)

end
"#{key} is missing"
I18n.config.missing_interpolation_argument_handler = Proc.new do |key|

You can suppress raising an exception and return string instead:
== Example:

if you don't care about arity.
are the same as for MissingInterpolationArgument initializer. Use +Proc.new+
object that responds to #call. The arguments that will be passed to #call
Sets the missing interpolation argument handler. It can be any
def missing_interpolation_argument_handler=(exception_handler)
  @@missing_interpolation_argument_handler = exception_handler
end