With Prism Mac, how can I write graph titles, labels, etc. in a language other than English?

For inserting an occasional Greek or international symbol, use the insertion button on the toolbar:

For using other foreign language in Prism Mac follow the steps:

Step 1. Click 'Language and Region' icon in System Preferences

Step 2. Click "+" to add a language, and select the language to add.

Step 3. When prompted, say that the new language is the preferred one. Then drag it in the list so it is on top.

Step 4. Reboot your computer so the langauge choice is enabled. 

By default Prism uses Arial and Helvetica Neue fonts which do have glyphs for many languages.
If you want to set different fonts, invoke Prism preferences, and change the defaults on the 'View' and ’New Graphs' tabs.