Lots of great suggestions in the replies here. Like most things in Marketo, there are a lot of potential avenues to accomplish this. I love segmentation (suggested above) particularly for things like auto-reply or trigger emails, but for one-off batch emails I occasionally prefer just creating a separate email asset for each language. This way I can specify at the Smart Campaign level, rather than at the Segment level exactly who should be receiving which email (which has come in handy when my subscription's segmentations don't quite align with the intended breakdown for the send). I also find triggering the correct follow-up campaign and analyzing the email analytics a bit cleaner this way -- the tradeoff is that you do have a bit more overhead in your program.
If you have local users in Marketo doing things like edits/approvals, this can also take away any potential ambiguity around exactly where they should be making their edits -- dynamic content may not be super familiar to them depending on their level of comfort with Marketo.
At a strategic level, I find this works best when you are able to have someone internally who can review each of the localized versions -- they're going to be much more familiar with the terminology your business likely uses than any localization service provider would be.
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