See my update as to compile-time vs. run-time.  IOW, when you write

BEGIN {
   setlocale LC_NUMERIC, "german";
}

$n = 1 / 2;
print $n . "\n";   # 0,5
print 1/2 ."\n";   # 0,5
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you get localized output in both cases, because localization is already in effect at compile-time.