I came across some interesting tips on multilingual SEO from Maria Nikishyna of Search Engine Journal this morning.
Stick to only one language per page
Avoid side-by-side translations
Use the same language for all elements of the page: headers, sidebars, menus, etc.
Language-specific extensions are often used on multilingual websites to help users (and crawlers) identify the sections of the website they are on and the language the page is in.
Crawling and Indexing
Avoid redirects based on user’s perceived language: they could, in fact, prevent both users and SEs from looking at more pages on your site. Keep the content for each language on separate URLs
Cross-link page by page
Like anything else in internet marketing, whether its SEO, social media or pay per click advertising, its important to start with the end result in mind and be consistent as you build your site out. Get the basics right and move forward from there.