What are these numbers for this element name? I have a part inventory for a set and it has some axles with numbers next to them. This is not a technic set, but I imagine the numbers are the same as in this question - LEGO Technic part Numbers 

But one of the axles has two numbers (5,5) - is this a comma because of the locale in which it was purchased uses the comma as the USA would use a (dot) for a decimal? Or does it have something to do with the fact that the collar on the axle isn't at the end?
Bricklink lists it as


  Technic, Axle 5.5 with stop


The blue number is the length of the axle. It is mentioned in the part list to distinguish axles of different lengths in a set. 

In this case, the length is 5.5, so yes, it is written as 5,5 because of the locale.