I'm tempted to goad you into being young, wild, and reckless and do it as an epub3 book. This is the kind of project where I find the limitations of epub2 most enervating. Do it in epub3, on the other hand, and the possibilities are as near to boundless as makes no difference... Tweaking the stylesheet to either have the text side-by side, or switching language for a single paragraph – the choice is yours, or rather, your reader's. Always assuming they can find an epub3-compliant e-reader, of course...
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