Indeed, this is frustrating and absurd. The article 'Do I need a language pack or language interface pack?' At says, 'Or to buy a language pack, please go to or.' And both those links go to the same page, for Office 2013.
FWIW, it appears that what you need is not the language pack but the Proofing Tools Kit, which includes proofing tools for all languages. If you google for Office 2010 Proofing Tools Kit, what you find from Microsoft is a series of service packs, but there are numerous sites that claim to offer the kit itself for download (free in most cases).
Although I would view these sites with caution, there doesn't appear to be any official channel for getting this package. Microsoft MVP (Word) since 1999 Fairhope, Alabama USA http://ssbarnhill.com. Indeed, this is frustrating and absurd.
The article 'Do I need a language pack or language interface pack?' At says, 'Or to buy a language pack, please go to or.' And both those links go to the same page, for Office 2013. FWIW, it appears that what you need is not the language pack but the Proofing Tools Kit, which includes proofing tools for all languages. If you google for Office 2010 Proofing Tools Kit, what you find from Microsoft is a series of service packs, but there are numerous sites that claim to offer the kit itself for download (free in most cases). Although I would view these sites with caution, there doesn't appear to be any official channel for getting this package. I agree this is at the least completely absurd.
Any other software package, for office, browser, non Microsoft offers this for free, without any hassle. So, know I just type in my text in a Google word document and copy past this in an Office document. 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. 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,. 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. https://jay-freedman.info.