The Differences in British/Canadian/Australian/American Conventions

Spelling

Check out this Truly Canadian Dictionary of Canadian Spelling (compares Canadian with American and British spellings).

This Wikipedia article is a great in-depth look at the different spelling conventions between British and American English. 

Here's a handy spelling comparison chart for all different versions of English, including the UK, South Africa, and Australia.

This post from Editor Australia does an awesome job explaining Australian spelling conventions.  

Punctuation

Here's a great comparison of British and American punctuation from Cambridge Dictionaries Online.

Here's another comparison of British and American punctuation from The Punctuation Guide. 

Macquarie Dictionary has a great resource section on Australian punctuation. 

Grammar/Usage

This resource from One Stop English provides a solid overview of differences in American and British grammar/usage, including tips on prepositions, past tense forms, and contractions. 

The Writer gives a quick general overview of the main differences between American and British English. 

This article from Proofreading Acadamy provides a quick overview of Canadian English.

Miscellaneous 

This list from Writers and Editors stockpiles a TON of different resources on spelling, grammar, and style you can browse through. 

A note about regional English conventions and style guides: Something tricky that's come up at Kibin is how to handle regional English conventions in conjunction with style guide rules; e.g., when to use single vs. double quotation marks, oxford commas, and punctuation inside or outside quotation marks if a user has selected a specific style guide and a non-US English option. The research (read: lots of googling) we've done hasn't turned up any definitive or authoritative results from the APA, MLA, or CMOS on the subject. 

Since we haven't been able to find much authority on the matter, the best advice is taking cues from the regional conventions the writer has included and ensuring consistency throughout the document.

Let's say you've picked up a document marked APA with Australian English, for example. The writer has included single quotation marks around all but a few of their direct quotations, they've got some missing Oxford commas, and use Australian spelling conventions. In this case, you might add the Oxford commas (since APA requires them), change the few double quotation marks to single, and ensure Australian English spelling conventions are used throughout since that will give the writer the most consistency within the document. 

If it's not fully clear which preference the user is leaning toward (sometimes they write "colour" but also write "favorite"), its fine to simply address this in a comment or change the document to the style of English the user selected. It's very rare to see user complaints about the intersection of style guides and regional language preferences, and remember that we've got your back. 

The TL;DR? Consistency is key, follow the author's lead, and use your best professional judgment. :) When in doubt, you can always send a quick message to the author via the Messaging feature to confirm their language choice.