It might be interesting to offer the reverse: incremental translation of source material that's in the reader's known language, into the target language.

That is, simply by switching into the mode, each page the user visits would have some of its words translated to the target language. They'd have distinctive styling, and a simple hover or click would show the original word. You'd always be reading a mosaic of both languages

As they hover/click for clarification less often, more and more words would be target-translated... building the new language vocabulary over time.

Of course, this doesn't teach the new language's syntax/ordering... but even that could maybe be incrementall mixed-in over time. Perhaps when there's a clear 1:1 sentence map, a sentence could (with some probability linked to how many words have already flipped) flip to the target-language's order-of-presentation, even if most of its individual words are still native.