I just noticed that in Computer/Library Dictionaries, I have three Japanese plugins. I have not a clue of how they got there:

1. Shogakukan Daijisen.dictionary (44.8MB)
2. Shogakukan Progressive English-Japanese Japanese-English Dictionary.dictionary (66.9MB)
3. Shogakukan Ruigo Reikai Jiten.dictionary (5MB)

When installing my OS, I was careful not to install "foreign" languages. Well, or so I think.

In System Preferences/Languages, I only have US English and English.

MS Office is known to install "unusual" components. Might MS be the culprit? At any rate, is it advisable/safe to delete them?

Your thoughts, musings and guesses most welcome.

foreign languages installs individual language translations for menus etc in each application. It doesn't completely skip the actual internal support

10.5.8 here. Have the same dictionaries in. PrefSetter can open a plist in one of them and it states that Apple is the manufacturer. I don't think, therefore, that they are related to MS. Maybe the system thinks that they are essential...

It seems a bit unusual that Apple would do such, especially without my invitation, but more sleuthing indicates that is indeed the case.

Any guesses what would happen were I to delete those plugins?

You could put them on the Desktop for a while and see if it makes any difference. Personally, I would not mess with them. I do remember the famous/infamous 4 Japanese fonts that MS decided to make indispensable for Powerpoint to work. Without them (I deleted them thinking that I would never use a Japanese font) PPT would hang on opening... Restoring them was no small feat.

Well... I deleted them from my very infrequently used Snow Leopard installation and haven't run into any issues... YET. (After all, it's not like they're fonts; they're only dictionaries.)

Edit: What the hey! I'll be the guinea pig and delete them from my 10.5.7 installation too. (Done, and OS X didn't even ask me to authenticate, which gives us some indication of their importance or lack thereof.)

They're part of Dictionary.app.

Open Dictionary.app and look in its Preferences. You can choose which installed dictionaries you want to see, and drag them up and down in the list to specify in which order definitions will appear if a word has definitions in more than one dictionary.

Um, they're 44.8, 66.9, and 5 MB respectively, for a grand total of 116.7 MB. Are you really that short of space? If so, by all means uncheck them in Dictionary's preferences and then delete them. The only consequence will be that you won't be able to check them again (and I have a hunch that'll be just fine with you).

For me in 10.5.8 they're 156.4, 135, and 9.9 MB, totalling 301.3 MB, but that hardly dimishes your point.

In case they later needed, I moved my Japanese plugins to a new/separate folder on a backup volume.

I presume that because they are just plugins they can be returned by merely dragging & dropping.

As with others, I have yet to see an adverse consequences from their removal, though time will tell...