tl-setup, and also to some extent tlinstaller, uses a lot of convoluted strings that do not adhere to the GNOME HIG writing style: https://developer.gnome.org/hig/guidelines/writing-style.html The main points would be to shorten a lot of strings, use a more neutral tone, and have consistent capitalization. Note that strings are shared between GTK tl-setup and text-mode tl-setup. Strings that fit in the graphical GTK interface might not fit perfectly in text-mode.
Also see these notes about typography: https://developer.gnome.org/hig/guidelines/typography.html
