I'm upset that there are 700+ courses across 70+ languages, but none is my language, Esperanto, the most successful international auxiliary language of all time, with an estimated 2M-5M speakers worldwide. I'm unable to select Esperanto as the language I want to learn, nor as the language I already speak. This is the kind of thing Esperantists have come to expect of language learning resources which support a low number of languages, but when there's an app which supports 70+ languages, and Esperanto isn't one of them, it feels like a deliberate omission.
There are tons of great reasons to learn Esperanto, but one is its propaedeutic value: learning Esperanto for a year, and then French for a year, leaves you knowing more French than if you had studied only French for two years. Some cite the ease of learning it (I learned it to a conversational level in about two weeks), and some the intuitive grammar (only 16 grammar rules, no exceptions). Whatever the case, the value of learning Esperanto is huge, even if you have no plans to become active in the Esperanto-speaking community.
It's great that Lairner allows one to learn endangered languages and minority languages—I'm learning Irish, for instance, which only has ~200K native speakers, most of whom are in Ireland. Learning Irish (and other Celtic languages) is immensely mind-expanding, and it unlocks a rich literature and culture. But the original literature and culture of Esperanto is even more vast, rich, and international, and is not restricted to any geographic area.
Another issue I want to bring up is the unnecessary conflation of language with geographic region. In Lairner, languages are given flags of nations, as if there's a 1:1 map of language to country. That is a really bad assumption. Languages go beyond the borders of nations. Just think of Basque, for example. Or Esperanto. (And Esperanto has a flag!)
A Turkish speaker can't learn Basque, by the way, using Lairner. Basque is not a language which is supported at all, as far as I can tell, and Turkish doesn't seem to come up as a possible L1 language for most languages. The site says "700+ courses. Every combination." But there is not every combination. To learn Irish, for instance, you have to already know English or German. There's no Chinese -> Irish, or Japanese -> Irish.
If it were me, I'd feature Esperanto as the language everyone should learn first. Then I'd ensure that, using Esperanto, you can learn any other language. That way you wouldn't have to write lessons for every language pair, only X -> Esperanto and Esperanto -> X.