As an ESL, I find manumitted not transparent at all. Orphan is a more common word, and even if you don't know the specialised meaning, you can grok it from context much more easily.

I find emancipate easy to understand, but then it's a Latinate word, and I'm a French-speaker. I'm not sure it would be clear to a speaker of a non-Romance language. I do like "set free" :)