The "Unlocalized text" simply means that it couldn't find text corresponding to that id. Localization is the term used when you create a different language version. The message is there to help the localizers find lines they missed.

Ids on computers are often case sensitive. In other words, Helot is a different id then helot or HELOT.

As to a unit limit, I'm pretty sure there isn't one or at least no one has reached it.