You won't loose much of your user base if you allow "attack/helicopter" as an acceptable value for the field.

That said, whenever I see this kind of "feature" added, I wonder if the authors (and I just wonder, I'm not saying they don't) are taking into account that it doesn't work in every language.

English is a gender neutral language, for the most part, except for pronouns, possessive adjectives and a few other words. Italian, for example, however, is very much not gender neutral and grammatical gender is conveyed even when no pronouns is used (and it's easier than it is in English to write sentences that don't use any pronoun).

This really is about grammatical gender, not pronouns (even in English there are gendered words other than pronouns). Neopronouns simply do not work in some languages. They do not work in Italian because other words are still gendered, so you would refer to a person with either masculine or feminine words even if you were to introduce neopronouns. 

 Exactly this. In Vietnamese pronouns are determined in large part by how old you are versus how old the person you're speaking to is, and many other languages have pronouns that cannot be determined without taking into account both parties in the dialog. A more global solution would be some sort of open ended "how can I address you" field, a sort of sub-bio.