We’ve recently finished building an Umbraco site that needed to support six languages: Arabic, Chinese, English, French, Russian and Spanish, and there were a few interesting challenges along the way 
We’ve written up some of the things we learned around culture variants, shared vs culture-specific properties, Umbraco translations, RTL languages, and handling content that’s automatically synced from an external source and needs to be machine translated.
One of the biggest takeaways was that multilingual content isn’t really one problem, different types of content often need completely different approaches.
Hopefully it’s useful if you’re building (or about to build!) a multilingual Umbraco site.
Building a Six-Language Website in Umbraco: What We Learned
Would be interested to hear how other people are approaching multilingual builds in Umbraco too, particularly where automated/external content is involved.