Mac is more fluent than ever. MacOS Mojave adds UK English, Australian English, Canadian French, and Traditional Chinese for Hong Kong language options; improved maps for China; and romanized keyboard input for Japanese.
The US International keyboard layout used in Windows differs considerably from Apple's US International PC layout on the option/alt level. You can get a test version of one for Mac which conforms to the Windows layout here.
Among the many 3rd party screen keyboards available for iOS, I recently found one called PADKEYS that imitates the normal hardware keyboard layout, with a number/punctuation row at the top, an option key for symbols and accents, cursor keys, etc. Covers English, Danish, Dutch, Finnish, French, German, Italian, Norwegian, Portuguese, Spanish, and Swedish.
In MacOS 10.13.2, it appears that you cannot switch directly from Chinese/Japanese/Korean to Cyrillic/Arabic/Hebrew/Greek keyboard layouts. In order to get the correct results, you must first switch to US or another Latin input source.
It's possible other keyboards are involved, but these are the ones where I have seen complaints and tested myself.
Hopefully this bug will be fixed in a future update.