How can I type the long a in Si_a_n, with the English keyboard? EDIT: of course there is Alt-0226 for â, or i can copy/paste the word "Siân" from the prompt. Anyone have easier/better ways?
On macOS you just choose the built-in Welsh keyboard layout - same as the British one, but with modifier keys set to the common accented letters. On iOS, and I think on Android, you just keep a key pressed for a bit longer to bring up a choice of accented variants for that letter.
On Windows there may be a Welsh keyboard buried somewhere. This has some info - http://symbolcodes.tlt.psu.edu/keyboards/index.html - which might point you in the right direction. You could also look on the tech support websites of the Welsh universities, and I think that Hawaii and Brigham Young unis in the US also teach Welsh.
Not all computer fonts include ŵ and ŷ. I usually use Bookman Old Style which has them if you get a recent version of the font - it comes with macOS and also Win10, I think. Common fonts such as Arial, Palatino and the various Times fonts should have them, too.
On windows you can install the extended UK keyboard which allows you to easily type accented letters including the "alt gr" key and "6" to created letters with a circumflex. To install it, go in to "all settings", "Time and Language", "Region and Language", click on "English" and then "Options", "Add a keyboard" then scroll down to "United Kingdom Extended". If you need any help with how to type certain accentes needed for Welsh once you have it installed you can ask me. (This is all for windows 10 sorry, I don't know if it's available for earlier versions).