Reproduced in this version:
Confluence 5.8.8 and 6.2.2
Google Chrome: 65.0.3325.162
Our customer recently upgrade their Chrome browser. They started to see this:

Create a new page in editor, insert a table and start to edit table header.
When input Chinese, the first letter is not treated as PINYIN, but left as english letter.
Things to observe:

When I type in other area on the same page within Editor, it is fine to type Chinese in PINYIN.
When type inside table cells other than table's column header, it is fine too. Could be something related to the headers?
And It is fine in Safari as well. So only latest Chrome browser have this problem.
https://community.atlassian.com/t5/Confluence-questions/Error-Chinese-text-input-in-Confluence-when-upgraded-to-latest/qaq-p/754115#M101637

This issue is discovered in server, then also confirmed in Cloud.
https://jira.atlassian.com/browse/CONFSERVER-55226