From plugins/numtheory/numtheory.c:232:

static char *help_nt_mu = {
	N_("@FUNCTION=NT_MU\n"
	   "@SYNTAX=NT_MU(n)\n"
	   "@DESCRIPTION="
	   "The NT_MU function (Möbius mu function) returns \n"
	   "0  if @n is divisible by the square of a prime .\n"
	   "Otherwise it returns: \n"
	   "-1 if @n has an odd  number of different prime factors .\n"
	   "1  if @n has an even number of different prime factors .\n"
	   "If @n=1 it returns 1\n"
	   "\n"
	   "@EXAMPLES=\n"
	   "@SEEALSO=NT_D, ITHPRIME, NT_PHI")
};


This message has an "ö" in it. This is unfortunate because this message
will break for every translation that doesn't use iso-8859-1. There can
only be one encoding of a po file, and thus msgid:s with iso-8859-1
characters and msgstr:s in another encoding don't match. The result is that
this message can't be translated.

There are two solutions:
1) Replace the ö with ASCII characters (you can always add a comment
immediately before the message that explains how "Möbius" really should be
written)
2) Make this C file UTF-8 and demand all translations to be in UTF-8. Then
you can use "ö" and "µ" and other non-ASCII characters in the messages
whenever you want to.