TabbedLanguages rewrite

I'm not sure what you mean by the intent regarding sortCats... The function goes through the categories placing them into language sections, moving on to the next language section if the category name begins with the name of the next section's language, except if the category also matches /letter\snames$|script\scharacters$|mythology$/, which, afaik, covers all the exception categories that start with language names without being related to the language. (Example: "Category:Greek script characters" begins with "Greek" but doesn't indicate the start of a Greek section.)

…and the categories are guaranteed to be in the order in which they are rendered on the page? In that case, I need no further explanation.