What version of InDesign are you using?
What kind of edits are you trying to perform?
You can read Japanese, right?
You can edit files from one localization of InDesign in another. You can start a document in Japanese ID, then I can edit it in the Middle East Edition of ID, and then hand it off to someone running the, I dunno, French edition of ID to finalize it. Either you're trying to do something for which there is no UI tool in your version of ID (possible, but not too likely) or you are misreading an error message and don't have the problem you think you have (extraordinarily likely). I would bet that someone is using a newer version of ID than you, and you're reading the incompatible-version error as "I can't edit Japanese files in English ID."
But I might be wrong. So can you fill in a few details for us?
I'm using CS6, which as far as I know is the latest version. The translator I'm using also uses CS6.
No I can't read japanese, but I was given text to copy-paste into this indesign file from a word document. But I haven't even gotten as far as trying to copy paste because of the problem below:
What has happened is I've given her the english indesign file, and she's translated it to Japanese text. Now when I try to select a sentence in japanese to delete or add to, I can't do it. The text cursor just sits there blinking, and moves neither left nor right. I can only assume it's because my english indesign is incompatible with the japanese characters, which is why I'd like to download a free japanese trial so I can select, delete, and add japanese text. No error message pops up at all.
Also to add, there's no issue with my indesign as other files given to me by this translator with english characters are able to be edited.
Your translation provider converted the text to outlines - to an image - to keep you from revising it, and/or damaging it. Although your install of ID can render Japanese text just fine, it's all too easy for someone who cannot read the language to ruin it without meaning to do so. (By, for example, pasting in text and reformatting. Line endings in Japanese are a bigger deal than in English, and there are no word spaces to make wrap rules easy for a non-reader to figure out.) You don't have the tools to edit Japanese text exposed in the GUI, but if someone sent you Japanese live text, you could see the J composer applied, and all of the Japanese-specific layout rules in the paragraph styles summary.
Long story short, if you want to revise that Japanese text, you need to get it rekeyed, or to return to your translation provider for the revision.
(CS6 isn't the latest version; there have been two editions of Creative Cloud since then. If you were a subscriber, you could download the Japanese version, I think, but if I am correct, it still wouldn't be able to edit the outlined text. You can check my hunch that your text has been converted to outlines by selecting it with the direct select tool - the white pointer. You should be able to see the outlines yourself.)
Darn, what a pain. Oh well, thanks very much for your help, Joel.