How can I add fonts to my Android phone to view characters that are appearing as X boxes?

I am frustrated with the apparent inability to figure out which charset or language I need to install to view characters that appear as “X”s on my Samsung GS6 phone.

For example: On Twitter, when I use the app on my phone, someone’s name appears entirely in Xs. I assumed it was a multichar language like Korean or Japanese (both of which I have installed) but it turns out it’s just bold face Roman. (Which I see when I pull up the user via Twitter’s web interface on my computer.)

Other times, I am sure it’s an emoticon or such that someone sends that doesn’t show properly.

How can I know which languages or icon sets to install so I can view these?

(Note, my computer, a Lubuntu machine, usually displays things fine. So this is a phone question.)

Thank you so much for helping with this. Driving me nuts.