Note: the spell checker still has some suspicious words in it. Over the next few weeks, I will add more words and take out the incorrect ones.
As Dorothy said in the Wizard of Oz, "There's no place like home." The truth is, you always had a spell checker in any language you can write in in Word. All you need is a large document with mostly correctly-spelled words. If you set up a new custom dictionary, you Add new words to the custom dictionary, which becomes filled with correct words.
I haven't exactly done it this way but this is essentially the approach to using the spell checking files provided here. The indo.zip file contents three files: indoaa.dic, indoab.dic, and indoac.dic. These should be unzipped and copied into the same directory as your other custom dictionaries. For my set-up, the files are in c:\Program Files\Common Files\Microsoft Shared\Proof but yours may be different. There is a good explanation for setting up custom dictionaries at Proofing.com.
The custom dictionary is separated into three files because of a limitation in Word97 that there can be no more than 5,000 lines (or words) in a single file. There is no limit on the number of files, though. I split the 11,000 word list into three. indoaa.dic and indobb.dic both have 5,000 words each. Indoac.dic has about 1,000. The trick is to set up all three as custom dictionaries. Under Tools > Options, select the Spelling & Grammar tab. That should bring up a box that looks like this. Select the Dictionaries... button and check the boxes next to indoaa.dic, indoab.txt and indoac.txt. The dialogue box should look like this. Leave the language associated with the files as (none) or the same as whatever language you currently have a spell-checker for.
Back in the Spelling & Grammar tab, make sure that the file name that appears in the selected window (the one that looks like this) is indoac.dic.
Since we don't have a main spell dictionary (like mssp2_en.lex for International English) for bahasa Indonesia, we fool word into thinking that the words in our custom dictionaries are special words in English. Don't set the language to Indonesian, leave it at the default language.
When you check spelling now, the spell-checker first looks the word up in its 137,000 word list of English (if that's your default language) and then in our 11,000 word list of bahasa Indonesia. It even suggests bahasa Indonesia corrections, along with some English ones if there are some words in English that are close.
The grammar checking is sometimes handy, too. If finds punctuation problems, long sentences, irregular capitalisation, etc. You can turn this on and off from the Spelling & Grammar tab of the Tools > Options dialogue box.
When you add words to the custom dictionary, they will be added to indoac.dic. You can edit these files yourself to take out words that are incorrectly spelled. Also, you can add new words by typing them into a blank document and spell checking it. When Word finds a word it doesn't know, you Add it to the custom dictionary.
This approach may be slow on an older computer. My 200MHz Pentium sails through the words with no great delay. I have found hundreds of spelling and punctuation errors in official Indonesian documents.
Best of luck! This is not a simple procedure. I hope it is worthwhile.
If you get the chance, send me your indoac.dic file after you have added a few hundred words. Please let me know if you have any problems with the installation. I will try to explain it in more detail.