For the English landing page, I'd suggest translating from another language into English, rather than from English to Chinese. I assume you built this app for yourself so you can watch technical videos in English with Chinese subtitles to help your understanding. But when you market this to others, you need to show how the product fits the customer's situation, not yours.

I'm not sure about the API key setting. Does it mean that whoever uses your program needs to get their own key for a translation service, which will bill them for their API usage? In that case, you really need to explain this and how to get one. Not everyone knows what an API even is!

However, I think it would be better design to handle the API keys on your end and simply give them a drop-down menu for choosing a provider. You get billed by the provider and then you bill them with a markup.

1 million characters is kind of a lot for a free plan. Who translates so many subtitled videos per month that they'd hit that limit? Only professional subtitlers come to mind, and they might be able to pay a lot more than just $9. If you want to make a profit, you should probably rethink your pricing strategy.

I think you might be able to offer your service to subtitling groups in China, but I'm not sure how much those would be able to pay. (I guess many are hobbyists who don't make any money.) You could try contacting some of them to get a feeling whether they'd find machine translation useful to speed up their work, and how much that'd be worth to them.

EDIT: Found https://1c7.me/subtitle-translation-helper/ Since you're already targeting subtitling groups, I guess you know what you're doing.