2 keke_eo: first you have to decide what do you want from your app exactly and then you just ask if there any way to get something you need. I don’t think such APIs that the one mentioned above have to be added to the list. They bring less information.
By the way, I missed the “http://www.lingq.com/api_v2/language/lingqs/” (without the content_id) in the list of APIs and that is the reason of my stupid question in the other topic.
Is you language level flips back and forth every day?
I think this API request would be enough for the app you want to implement. Are you afraid of the amount of traffic you have to download? If it is because of traffic - make a cache. If it is because of device’s memory - don’t worry, the linkqs in json format do not eat much memory. I have, for example, about 240 words in my vocabulary for now and it takes only 61 Kb in json format to download. And it is in UTF-8 ! Don’t pile the words into vocab, remember them from time to time 
Let’s be realistic - people will never be satisfied with any amount of API features in any system, it’s a human nature )
If you find anything as abusive - I’m sorry. I had no intention to abuse anybody.
I have another suggestion: I think it is essential to add to the “http://www.lingq.com/api_v2/%language%/%lesson_id%/text/” API request the feature to reply with the text marked with “known/unknown”, so that at least the feature enabling users to mark unknown words as linqs can be implemented in mobile app as it is now implemented on the site.