If your primary purpose is to type Turkish without adding one of the Turkish keyboards provided by Apple, then this can be done by Adding the
ABC - Extended keyboard-map. All Turkish letters are easy to type with the "
option" key, add this keyboard map, and view the layout using
Show Keyboard Viewer in the keyboard (change keyboard icon) menu.
Specifically - I was not sure whether
"İ/ı" are available on this layout, but after some experimentation - they are there!
"option-w" - is adding a
'dot above' a letter, however when pressing
"i" - it actually removes the dots (there's well thought out design for y'all!). [note that specifically ı/İ - are absent on the "long press" mode you're referring to, but using the
option key to type is much faster, I regularly type Polish, Swedish, Finnish and now Turkish this way, without even having to switch layouts, and I find it easier than the respective maps] [ Adding letters to the long press mode - is possible as mentioned in in @Tom Gewecke's reply]
I'm using MacOS
10.14, and generally use the
ABC - Extended for several languages. Gladly it also works easily for typing Turkish as well (which I've just started to learn, and use to type in Chat windows).
The current mapping is, before the respective letter press:
ç ş Ç Ş : option-c
ğ Ğ : option-b
ı İ : option-w
ö ü Ö Ü : option-u
â î û Â Î Û : option-6
Note that it's possible to press the same "
option-shift-(something)" after the letter, but the result is different - the first gives a "combined" letter, while the latter gives a "letter+combining character": for example "ğ / ğ" - are different (might be rendered a bit differently, the latter is "g + combing char brev ˘", in case of "
ı" - the latter option might render i with an extra combined dot: i̛ - which is probably not what you want.