I apologize for taking so long to respond here. I agree with everything GrammarCrazed says here. An apostrophe with "s" can only be used for possessives or contractions of "is" or "has":
Navi's been missing for a while, hasn't he?
I agree with what Ahmed says about Nazis, but not for the same reason. Even with common nouns ending with "i", the plural is almost always made with the addition of a simple "s"; hence "skis", "khakis", and "taxis" (although I've seen some sources that say that "taxies" is also accepted in BrE; I've never actually seen it used). The "-ies" suffix is generally only used if the singular ends in a "y"; hence "navies" is the plural of "navy", not "Navi".
There is an exception made for names of people, as with GrammarCrazed's example "Flahertys". However, this does not apply to other proper nouns. "Nazi", "Tory", and "Wobbly" are all proper nouns, referring ot members of political parties, and their respective plurals are "Nazis", "Tories", and "Wobblies". Also, in 1815, the Kingdom of Naples united with the Kingdom of Sicily to form the Kingdom of the Two Sicilies, and before their reunification in 1990, it was correct to refer to East and West Germany collectively as the two Germanies.
We have a highly respected member on this forum whose name is also Navi. I'm surprised he hasn't weighed in on this.