It's hard to express 'x' to non-Chinese speakers...
These websites might be useful:
So, sonika with her existing phonemes actually cannot do proper Chinese, but talented producers can make her sound like Chinese. You have to be a native or really fluent in a certain language to do this, and the result varies with voicebanks and producers. Sometimes missing phonemes don't bring any problem...
Off topic, just for reference, a perfect KAITO mandarin by Teka-P and Shizi-P:
But extra voicebanks never hurt.
On topic, I never expected Tianyi to come with an English bank, but who knows. Remember Shan Xin talked about a 97-page book for recording, a single voicebank by no means will need that much.
And the voice provider's Japanese doen't have any obvious accent, I'm not sure what will happen with the voicebank but it's good news.