There have been various suggestions and conventions. The most common is "they", AFAIK followed by "ey" and "ze". Some other languages already provide them, or are much easier to work with.

Someone who makes an effort to use neutral pronouns in a culture-neutral Japanese will succeed much more easily than in English (but culture-neutral Japanese is rare, and most culture-influenced Japanese involves arcane multilayered complex n-dimensional politeness levels / attributes / tags / markers / etc. that very often depend on the gender of the speaker, who they're speaking to, and who they're talking about - all as separate variables).