AFAIK the two APIs (v2 & v3) exist side-by-side. Depending on what you want to do, you will have to call both APIs.
I have asked LingQ multiple times to provide documentation, but it has been deemed low priority and v3 was introduced 1.5 years ago; in short, don’t think this is going to happen.
To be fair most parts of the v2 API are undocumented as well. Things seem to work mostly for me, but I only use it for primitive tasks like up/downloading, getting statistics and updating my lessons. Not sure what your plans are.
Anyways, if you get stuck you can always ask in this forum, there is a good chance someone else has faced the same problem previously.
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