On the Android print screen, I only see American paper sizes (Letter, Legal etc.). Is there a way to add or switch to ISO sizes (A4, A5 etc.)?
The issue is linked to the language you've set in your Android device.
Most probably you set the language as "English (United-States)" and with these settings you only get the American format in you "Paper Size" dropdown.
Therefore, if you want to have the ISO size but still keep your phone in English, you just have to set the language of it in "English (United Kingdom)" and your problem will be solved.
Settings > Language & input > Language > English (United Kingdom)
Close your apps and try again, in "Paper Size" you should now see the ISO formats.
I believe it's up to the developer to provide whichever print job attributes they want to support for their app. The ISO, Japanese, and other media sizes are defined in Android's API here and I think it would be up to the developer to use them.
The developer can modify the print dialog for the user via a custom print document adapter that can include a set of default print attributes that are appropriate for what's being printed, and I bet this is where they'd include the ISO sizes.
Unless someone has a better solution (and who knows-- there could simply be an option somewhere to change based on location or language that I just don't know about), your best bet may be to contact the developer of the app you're using and ask them to add the ISO sizing attributes to the list of default print attributes.