Based on the level of writing, grammar, punctuation, and so on in this post, I am going to assume English is not your first language. Based on that, my first inclination is to believe you may be having a miscommunication with your manager (if you work in English). It is not feasible to build a piece of software that licenses for $225 in 8 weeks as an intern with no team or guidance, it's simply not. So you should go to your manager and ask very specific, very pointed questions. Here's how you should go about it:
Firstly, explain to your manager that English is not your first language, and it's possible you've misunderstood some requirements. You would like to ask him some very specific questions about the project, and you would like very short, very unambiguous, very simple answers. The questions are:
Is this a project he is intending you to complete, or is this a project he is intending you to start, which will be completed in the future by someone else? If the latter, how much work is he expecting you to complete? Is there a benchmark or is it "just do as much as you can as best as you can"?
Is this project supposed to be written on your tech stack (JS/React) or on your mentor's tech stack (Java/Kotlin)? If the former, presumably the company has someone who is going to maintain this application once you're gone; can you have that person as your mentor instead of the person who is mentoring you now?
Who can you lean on for help with learning and mentorship in terms of coding skill? "Just Google it" is an acceptable answer for a full time employee in the upper-junior or intermediate level, but not for a junior engineer and CERTAINLY not for an intern. As an intern, your goal for being there is to learn something, and you want to know what it is you're going to be learning, how you're going to be learning it, and who's going to be teaching you. If any of those answers are "Google", then this internship is a waste of time.
Make sure you get very simple answers, and if any of the answers don't make sense, ask followup questions, don't just assume you understand, because probably you don't. Until you are absolutely 100% crystal clear, in no uncertain terms, of what the requirements are, keep asking followup questions. A followup question might be something like this:
You: Am I supposed to write this application in JS or Java?
You: But MENTOR doesn't know JS, how is he supposed to mentor me?
Manager: He's not going to be very hands-on with you.
You: But he's my mentor, how can he be my mentor without being hands-on?
Manager: He's mostly doing his own work, you're mostly doing your own. He's tracking your work.
You: ... (etc)
You: You've asked me to reproduce a piece of production-level software which costs $225 for a license. Is that correct?
Manager: Uh, who told you that?
You: MENTOR told me that. Did he tell me wrong?
Manager: Yes, he told you wrong. We want you to build Fizzbuzz and a fun interface for it to show to the elementary school down the road.
You: That seems rather simple and I can do it in 1 day. What else am I doing for my internship?
Manager: Oh, we work with a lot of these elementary schools. We have more projects for you after that.
Ask all the questions you can and get all the answers you can to know the parameters of your internship. Then work to fulfill them.
Here's the other thing: As an intern, you're probably not being paid a competitive wage. You're likely making a significantly smaller amount than even a junior engineer. The reason for this is because you are not expected to perform at the same level as even a junior engineer. For even a junior engineer, their responsibility is to produce. For an intern, your responsibility is to learn, and secondarily to produce if you can. So you should act like that. Spend your time learning, take it slow, do a good job. Ask lots of questions and try to get as much feedback as you can. And absolutely foremost, do not work more than you have to, and do not stress yourself more than you have to. You aren't being paid enough to deal with this much stress. This type of stress is above your pay grade, so act like it. You will do what you will do, and that's what you will do. And if that's good, fine, and if not, fine. Either way, no stress.
As for converting to FT: Think about it this way: You're an underpaid intern who has no responsibility. They're treating you like this. How do you think they treat their employees who actually have responsibility? Do you really want to work for such a company anyway? Do you want to deal with this level of stress, and more, for the entire rest of your life? Is that enjoyable for you? Consider that, and then ask yourself: Is it worth it trying to work so hard to get FT at such a company, that will make you hate your life for the next 20 or 30 years you're working there? If not, then who cares if you get FT or not? There's plenty of companies to work at, you don't have to get FT at this one.