I'm not sure. We're using Elasticsearch. The query is made in this file: https://github.com/ipfs-search/ipfs-search-api/blob/master/search.js
As you see, we're taking the query literally from the input. But perhaps there are UTF-8 issues, or perhaps Elasticsearch has difficulties with Chinese. You might run the crawler on your own machine using Docker, then index a few Chinese files and play around with it. You might also look into any issues involving Elastcsearch and Chinese characters.