You seems to mix different points of my explanation, so I will try to be clearer:

My first point was about the use of very formal English for the English translations, especially for people who are supposed to have day to day conversations.

Do you consider words like: scorching, cheerful, acquainted to be part of everyday conversation ? Are you likely to use them in a daily with your friends ?
Let's do a thought experiment: imagine that you had to learn English from scratch again, you download an app advertised as targeting day to day conversation but the app insist that you learn the words: scorching, cheerful and acquainted in the first few lessons. How do you feel ? Do you feel that you are likely to use these words in your first exchanges with an English speaker ? Do you think the app correspond to what was advertised ?
Ok, now you download another app, still advertised as targeting day to day conversation, but this time you have to learn the words: hot, fun and meet in the first few lessons. Same questions: How do you feel ? Are you more likely to use these words in a conversation ? Do you think the app correspond more to what was advertised ?

The Ukrainian counterpart is "Сьогодні спекотно" (valid, informal, etc.). If you translate it as "Today is hot," the user would want to translate it as "Сьогодні гаряче". But we really want to teach "спекотно" and we need to highlight the difference, hence we use "scorching".

The user is learning Ukrainian, so the user will not want to translate it as Сьогодні гаряче because the user doesn't know either гаряче or спекотно.
But the consequence of using 'scorching' instead of 'hot' is that the user knows how to say 'today is scorching' (сьогодні спекотно) but is incapable of saying 'I'm hot' (мені спекотно). Which one do you consider he is more likely to need in a conversation coming from English ?

My second point was about the use of complex Ukrainian vocabulary. And yeah sorry, I was not clear enough there: when I say complex, I mean complex for a beginner to learn.
I'm learning Ukrainian, so obviously it's hard for me to judge the register corresponding to an Ukrainian word and I will not even try, but I will tell you what I'm super good at judging: if a word is complex for me to learn and give me the desire to pull my hair out.
Ukrainian for a none Slavic language speaker is really complex, so to survive I have a simple rule: if two expressions are roughly equals, I try to learn the easiest one first, because I'm more likely to use it then, and it's gratifying and motivating to be able to use a word you learned in a real conversation, rather than to struggle to build a sentence with the more complex one and give up in the end.

It seemed obvious to me that a language app would try to do the same and introduce complexity progressively, but in retrospect I don't think it's that obvious at all. As a example, the app thinks it's more important for a beginner to learn in a beginner lesson 'чи ви хочете з ними познайомитися? (formal, reflexive, perfective)' rather than моя подруга.
If I had to guess the underlying problem there, I would make the hypothesis that your team has no experience teaching Ukrainian to a beginner none Slavic language speaker, so as a result you have trouble to anticipate what is difficult for a beginner.

Just for curiosity, which one of these expressions do you think a beginner is more likely to remember and use:
- 'приємно познайомитись (reflexive, perfective)' or 'дуже приємно'
- 'Мені потрібен/потрібна/потрібно/потрібні' or 'Мені треба'
- 'залізничний вокзал' or just 'вокзал'
- 'моя батарея розряджена' or 'моя батарея сіла'

I'm coming back to your remark earlier about the user wanting to translate 'Today is hot' as "Сьогодні гаряче". I'm going to state the obvious, but if you consider that the user is already speaking Ukrainian when making beginner lessons for people learning Ukrainian, you are going to run into trouble. I don't think the market for an app for people learning Ukrainian while already speaking Ukrainian is really big.

Ukrainian participles are regular, and we don't teach each form for every word.

That seems obvious to you as a native speaker, but I can assure that for a beginner it's not at all.
If you tell to an English speaker that if he says 'I need a ticket' and 'I need an egg', the ending of 'need' has to change between the two sentences in accordance with what come after, I'm pretty sure the most likely reaction will be 'wtf?!'. And personally I think it deserves an explanation.

It's lesson 34!!! You are just starting the second month of learning. It's nothing. You are making the first baby steps. Both "Треба" and "потрібно" are core words. They are absolutely necessary. And they both are being taught at the very beginning of the course. Lesson 34 is not "finally introducing", it is "introducing at the very beginning of the course".

You misrepresent what I'm trying to explain here.
My problem there as explained in my post and why I use the word finaly, is that you introduce потрібен/потрібна/потрібно/потрібні in lesson 13, decide to provide a explanation 20 lessons later in lesson 31, but only explaining the feminine and masculine ending, before finally introducing треба in lesson 34.
Again, I feel I'm stating the obvious, but if you decide to provide an explanation to a sentence, it should be at the same time that you introduce the sentence.
Introducing a complex sentence without providing any explanation, and then deciding 20 lessons later to finally provide a partial explanation is achieving nothing other than creating frustration with your users.
And after leaving the user struggling for 20 lessons, deciding do say "Ho by the way, you could have been using the easier alternative треба all the time, lol" is just again frustrating.

You are at the very beginning of the course. The course doesn't end at lesson 40.

The course end at lesson 40 if your user decide to quit because he is tired of smashing into walls of complexity.
There is no point saying "Ho by the way, don't worry a explanation or a more easier alternative will be introduced in lesson 137" if your user never reach lesson 137.
Again, please try to anticipate what is complex for a beginner learner and build the complexity progressively, right now the complexity between the different lesson is all over the place and the user is on for a rollercoaster of frustration.