Why should I? Get a good dictionary and read it
To quote from the Random House DIctionary:
"a high-level language used to write computer programs, as COBOL or BASIC, or, sometimes, an assembly language."
To go deeper in the definition of language (which you may miss) in this context, again from the Random House Dictionary:
"Computers. a set of characters and symbols and syntactic rules for their combination and use, by means of which a computer can be given directions: The language of many commercial application programs is COBOL."
Note that it does not say "Product name". Many people (mostly beginner without a clue) make the same mistake for example thinking ASP is a programming langauge - it is not and never was (the ASP langauge below was actually replacable, but mostly Visual Basic was used).
So, finally there is common sense (get it) - even Microsoft does not claim Excel is a programming langauge, but does in fact fall back to VBA here (Visual Basic for Applications), which INCIDENTALLY is not even core part of excel. It is installed with excel, but it is also installed with some other software and there was a time (it is a retired product) you could license VBA for inclusion in your own applications.
You can read more on VBA at
http://en.wikipedia.org/wiki/Visual_Basic_for_Applications and to my check the version of the article is accurate (mind you, i did not check the dates). As you can see, it is an external system that you actually use when programming.
I personally find the amount of "willfull not knowing of facts" among excel people astonishing.