hi,

with option "Multi-lingual", what language is displayed by default, if there is no text for a particular language?

example: a page is translated in French and English only, selected language is "HTTP Accept Language" (but all languages are allowed) and there is Chinese visitor.

Presuming that the Chinese visitor did not have "French" in his prefered language list, English would be used if text for it exists. Otherwise French.

you mean that there is no default language with the option "Multi-lingual?

If the Chinese visitor has only Chinese in his language preferences (it has neither French nor English), what appens on ZenPhoto if there are only two versions (English and French) of a page ?

As sbillard said, the next language with content should be used. Since Zenphoto is in English originally English is the "main" default language. If you don't have content for it but for French, French should be used.