Without -q, emacs can enter po mode (provided by gettext-el), but
when -q is used, it does not. This is not consistent. The -q option
is described as follows:

    -q, --no-init-file
          Do not load an init file.

but this does not affect the loading of "site-start.el".

As a comparison, diff mode can be entered whether -q is used or not.

I can see that with -q, auto-mode-alist does not contain the config
for .po files that is defined in
  /usr/share/emacs/site-lisp/elpa-src/po-mode-0.26/po-mode-autoloads.el
namely:

 (add-to-list 'auto-mode-alist '("\\.po[tx]?\\'\\|\\.po\\." . po-mode))

Note: tests done without a .emacs file.