While this is great, I like to give a shout out to DeepL Translator [1]. I'm not affiliated with them but I like to recommend them to people who like to step out of Google ecosystem. I am using DeepL for about a year now and I mostly use it for NL EN, DE EN. So far, I never felt that the translation is off, or terrible, and if not better in some cases, it's as good as Google Translator. [1]: https://www.deepl.com/tra…
It looks useful but the lack of non-European languages makes me slightly suspicious, I wonder if their approach generalises to Arabic, Chinese, Japanese, etc.
The bigger problem is likely to be lack of training data. Unless they have a pile of cash to pay professional translators to produce a parallel corpus, the alternative is to scrape translations from the internet. Basically, crawl the same site multiple times with different Accept-Language headers and try to align the results. Crucially, this depends on an existing ecosystem of bilingual websites with high-quality human translations.
According to DeepL's website they're a spin-off of Linguee, who provide a search service for exactly that kind of parallel data. So before DeepL starts supporting any given language pair, you should expect it to appear in Linguee first. https://en.wikipedia.org/wiki/Linguee
Edit: It took me a while to figure out how to select a different language on https://linguee.com (Their UI seems broken using mobile Firefox Preview.) Appending /english-chinese and /english-japanese to the URL shows that they already support those two, and the alignment of translations appears reasonable to me. No /english-arabic, though.