Text objects in a project can be marked as "translatable". This is an option in a Text's halo menu. It is disabled by default. To make a Text object translatable, enable it.
If a Text object has translations, then switching Etoys to a new language will change the Text, too. Similarly, when loading a project on a machine with a different language, the texts will change after loading. (this mechanism is different from translating Etoys itself, which uses "PO" files)
To add a translation of your own to a Text object, you simply switch Etoys to your new language and then edit the Text. Then switch to another language. The new translation is stored when switching away from a language. When you are done, save the project.
You can see all translations in the "translations..." submenu of the halo menu for each of these text objects.
Hint: Some text objects (like in the Home project's clouds) are "locked" to avoid accidental editing. You need to unlock them using the halo menu, edit, then lock again.