Why can't you copy and paste? No, don't go through PDF, that sounds disastrous. Really, you should not have received a PDF. A Word document perhaps. You will need a Japanese font to set Japanese text, clearly. But you may have one already. Does the client specify a font? Be warned that some Japanese fonts costs many thousands of dollars and have limited licensing.
We have an expert on foreign languages here that hopefully will see your post and give you some advice. But in the meantime can you start by telling us what version of Indesign are you using and on what OS. Also, you mention a PDF that is already in Japanese. Is it already formatted the way you need it? If that's the case you could File/Place the PDF into Indesign. It will be placed as an image so you won't be able to edit it. If that's not the case let us know.
Honestly, I think it's time for you to job it out. If you are a current Creative Cloud customer, you can download Japanese InDesign with an English interface, and get access to all the tools you need to do a correct layout. However, you don't have any of the skills you need. Japanese layout is not like being an American and then tackling a French document. It's not hard to cover the basics of Where Well-Set French Type Differs from Well-Set English Type in a single post. But Japanese is entirely different.
Since we don't know much about the files, we can't even be sure that you can copy text out of the PDF and paste it into InDesign. You don't need a Japanese keyboard/input method installed to do that successfully. You do need a well-formed PDF and a font with the correct encoding. It's possible you already have that, but if you do but you can't read Japanese, then you should do a glyph-by-glyph proof when you're done.
If you do get valid text out of your PDF, you need to be able to turn on the Japanese composer because it's pretty much impossible to get the line endings correct without it. Japanese doesn't wrap like English, nor does it really hyphenate, so you have a 0% chance of a correct layout if you don't turn this on.
It's quite odd that the translator only delivered PDF. Perhaps the translator thought that the formatting was going to be done by someone who can't read Japanese and then was most likely going to break it some way? You should try to identify the application the translator used (which you may be able to do by looking at the properties of the PDF in Acrobat) and then ask your client to ask the translator to supply that source file? Or perhaps try to save raw text or RTF out of it to place into InDesign?
... I am reluctant to download a Japanese font at the risk of content translating incorrectly.
That is quite an ominous statement. The font has nothing to do with translating, and "applying a Japanese font" to English text does not translate it!
Anyway, InDesign comes with a set of high quality Japanse fonts (admittedly, my assessment of 'quality' only applies on the looks as I don't read Japanese).
I need Japanese InCopy with an English interface... I have downloaded the Japanese version of InCopy CC 2015...and now don't know what to do.
As I can't read all the characters...can you help?
I am writing a bilingual program with a Japanese teacher. We need to use ruby/furigana with an English interface.
I look for ward to your response.