Proofing tools for Office 2010

But I'm unable to purchase this product because their credit card payment form expects me to be living in the US demanding that I fill in which state I live in and not allowing me to select another country of residence.

GRRRRR!

As Jay has pointed out, what you say about Office 2013 is true (language packs are free), but the OP has 2010, so we're attempting to provide an Office 2010 solution.

I think the link in Office 2010 probably did originally go to the right page, but that page was appropriated for Office 2013, and then a new page had to be created to replace (restore) the original 2010 page. This was done as a result of our intercession with the product team.

Microsoft's policy on proofing tools has changed three times in three consecutive versions of Office:

• In Word 2007 and earlier, there was a single Proofing Tools package that was sold (both by Microsoft and by other vendors such as Amazon.com) for approximately US$100. It contained a large number of dictionaries and thesauri, most of which were useless to most purchasers.
• In Word 2010, the package was separated into individual language packs, each of which costs US$24.95. They are sold only through Microsoft's web site, at http://www.microsoftstore.com/store/msusa/en_US/pdp/productID.253665800. You must use your browser to go there, not the link in the Language Preferences dialog of Word 2010 (the link in the dialog still goes to the wrong web page).
• In Word 2013, the proofing tools for all languages are free. However, the Word 2013 language tools will not install in Word 2010.

The failure of the link inside Office 2010 to go to the correct page is a mistake, and I don't know whether there will be anything done to correct it.

The way to add an additional language to the proofing tools is to purchase and install a language pack. I don't believe the proofing tools have ever been free. Some Language Interface Packs are free, but they are offered only for "exotic" languages for which there is not a language pack.

That is up to you to believe or not, but in 2013 any languages(include exotic) can be downloaded and used for free(I mean proofing tools, NOT a language pack that customize interface of MSO itself).  I don't want to change language of  interface of MSO each time when I need to spell check files in different languages. It always was easy to keep a bunch of dictionaries for other languages without a problem

Right now when one will click inside MSO to link "not installed" it point to MSO 2013 instead of 2010.

It is a glitch or a way to force customers to buy 2013... or force to switch to Google docs...