I'll clear this on request.
The official en-US and my translation (ja-JP) uses zero for reset page zoom. It is natural, because it is same as the default Firefox key bindings for text zoom.
On my prospect, English and Japanese users, who uses en-US locale of Firefox with standard US keyboard and ja-JP locale with Japanese keyboard, repectively, don't have the problem. The problem is that pressing Alt+zero on main keyboard or Alt+zero on numpad don't reset page zoom. So it is defined alt+0 (zero) for them.
However, other people, who use their locales of Firefox and thier language/country-specific keyboard layouts, may find the problem as follows:
1) Some cannot reset it with Alt+0 (zero) on the main keyboard.
In some keyboard layout, 0 (zero) only can be typed with Shift ot Maj key. For them, you need to type Alt+Shift+0 (zero). It is somewhat painful and confusing (users may think it is unusable for themselves).
2) Some cannot reset it with Alt+0 (zero) on the numpad.
In some keyboard or environment, Alt+num0 (numpad zerp) just does not work. I don't know why, at present, but one possible reason is it is conflicting with Alt+number method in Windows (type 4 digits on numpad with Alt to get a European-specific letter).
These problem above is language- or country-specific, as my understanding, and I cannot do much for them because my resource is restricted. I only maintain the official en-US (as I am the author of PageZoom) and ja-JP (as I am the translator for it) locales.
For other locales, it is up to the translators. If you (a translator) find the problem above in your environment, you can change the key binding to reasonable alternative for your locale people. You can achieve it by modifying the entry in pagezoom.dtd. If you cannot decide what key is appropriate, you can leave it as default (same as en-US locale).
One demonstration: As for U.S. keyboards, "+" can be typed "=" with shift key, PageZoom also defines Alt+"=" for an alternative of Alt+"+" (+ and = for zoom in). For standard Japanese keyboards, "=" is replaced by ";" so that ";" is defined for the second keybinding (+ and ; for zoom in), which conforms the text zoom keybindings of official Japanese localization of the Mozilla Firefox itself.
Any other questions, comments or requests for clarifications?
By the way, PageZoom 0.2.2 is released public.
It is available on my page: http://www.memb.jp/~...zilla/pagezoom/
It will be also available on addons.mozilla.org after it gets approval by an editor.