Scootworks is correct.
If you use the ZOs designed string constants way (create strings, update it with newer versions, read them via function GetString) of translation then the highest version of the string will be used. So if your lowest version is in en.lua and will be loaded, and no other language file overrrides the same language constant with the same or a higher version, the en version will be used.
https://wiki.esoui.com/How_to_add_localization_support
If you define your own string tables without that Zos feature it depends on your code what is used!
if you always override the same table, the last loaded one will be used. So if en.lua is always loaded and no other ylicnetlanguage.lua file is found it will be using en.lua then.
If another client file is found it will only use the table entries of this one.
Exception: You can use setmetatable and point the index of your fr.lua table to the en.lua table entries to "fill-up" missing fr.lua entries with the ones from en.lua as long as the table in en.lua is not already overridden in fr.lua!