I have worked on some sites that have been translated into several languages, so I hope I can provide some insight.
How can you account for this and what are some good practices for content mockup?
Some of it is just common sense, like don't put any words in an image because then you can't translate it. Instead, store everything on the page as a series of strings, then have a corresponding file for each language. This is also called localization. And it should be architected into your code base from the beginning. This isn't exactly something to do later after you've created the site in English.
If a client supplies content are you to take the English version and go to Google translate and translate every requested language and design accordingly?
Google translate will not be good enough. Good translation is done by an expert in context. For example, on one page we were translating into Spanish, an editor explained to me that the gender for a specific word is normally feminine but because it appeared as a link on a page it needed to be masculine. This was way above me because I don't know Spanish, which emphasizes that a translator is essential if you want any amount of quality.
I know Japanese, and I know google translate is absolutely horrible in Japanese.
Do you design and code each .css file for each language if you are wanting the best possible result?
I wouldn't recommend this because it's just not scaleable. When you translate your content into other languages the biggest problem is you will have shorter or longer text depending on the language. For example Spanish is roughly a quarter again as large as the English. And Simplified Chinese takes up a lot less space. So I would include CSS attributes like "max-width" and "min-width" and QA your site with really long and really short paragraphs and titles to make sure it's versatile.
When providing a mockup to the client do you provide the translated edition or just the English?
In my experience and with my clients, I have gone through and done everything in English first but included the coding infrastructure to enable translations. I even publish the site. Then I go back with my client and work on translation. But if your clients know the other language (mine didn't) then I would ask them what they would like.
Is it a bad idea to not guarantee or require a sign-off for your design if you don't speak the foreign language in question?
If they provide the content, does it really matter? It would be smart to make sure you aren't doing design for a site with content you're not comfortable about, but you could plug the content they provide into Google translate to get a good enough translation if you are worried about that.
The best site I've seen that handles translation into 100 languages is www.lds.org. Use the globe icon at the top of the page to select the language. You'll notice that not all their content is translated into every language, so they have an advanced custom software to enable certain content in certain languages.