Found a possible issue in the reading list endpoint since it is not able to send the language variant when adding an entry.
For example, the Internet of things in Chinese is: https://zh.wikipedia.org/wiki/物联网
The display name is IoT技術 in Traditional Chinese and 物联网 in Simplified Chinese.
When saving these two articles into the same list, the endpoint will consider they are the same entry, and will only save/sync one article into the server.
Not sure if it is possible to have the RI team fix that by adding a new column for adding language variant.
Could be fixed by this PR: https://github.com/wikimedia/apps-android-wikipedia/pull/448