I’ve no idea how long it will take at the moment - though it may depend on whether you will be able to influence your fellow Mozillians Axel and Stas 
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Most of the work has been around in some form for about 9 months, so far we have managed to land the first FluentBundle implementation. I’ve had to rebase the patches many times, and realized that now the best thing is to tackle them one by one.
So:
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the first is PR 92. To move forward I think Axel would like a spec for the resolver, probably in the form of test suite that includes what errors are produced for various error conditions. I think this is also blocking PR 104.
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then there is the compiler branch, which is a large bit of code that may require some time to review. At the time of writing it is pretty much up to date with master and in a good shape, because I’ve updated it recently and it is relatively independent of the other things going on.
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finally the escapers. This is currently incomplete - I have only implemented it for the compiler
FluentBundle(which is what django-ftl uses), not the resolver. I did have an implementation for the resolver too, but with lots of changes on master it will be easiest to start over, and I’m not going to attempt that again until everything else is done and other bugs with the resolver are fixed. The whole subject of whether this is the right way to handle HTML will also need some design discussion - I think the last communication we’ve had on how to do this correctly are my comments here (see stas’s post that I quote).