It seems to me the best way to ensure the work is internationalizable is to have someone actually attempt a rough translation, and to use this phase to deal with the issues that show up as a result.
The alpha internationalizer should start around the end of Milestone 2; this will ensure that there is some text present to translate (and also that this person can have an account on the app), but also provide lead time for that work to proceed before I get to Milestone 5.
Jake, I'm assigning this to you on the theory that you can find the right person to do this work.
Other subtasks for this milestone can be spawned after the internationalizer and I have had a chat.
(In case it helps: I'm only fluent in English, although if for some reason you choose to internationalize into Latin I have a strong background there. I know enough about Romance languages and general linguistics that I can talk productively about many Indo-European language issues (I used to have some proficiency in Spanish and ancient Greek; super rusty in both now). Outside of those, I will find anything the internationalizer says *very interesting* but I won't have much or any background.)