Hi mate,
Yes, a Custom API option is being worked on and may be released as early as tomorrow :)
Thank you for the suggestions. I've looked around at different APIs and most do offer a free limit, but most seem to not offer a default rate limit. Instead the user has to set the limit themselves, which in some cases isn't straightforward for non-tech-savvy people. I'm not too keen on implementing APIs as such due to risk of having users blow way past their limit and blaming it on GameTranslate. Of course there are many things I could do to prevent this from happening, but it would need some careful consideration and be thoroughly tested.