<bing:TileLayer ID="BingLayer" LayerStyle="Road" Visible="True" ServerType="Production" Culture="fr-FR"
Token="{StaticResource BingKeyString}" />
The label is not generated on the fly but is "baked" into the image tiles. So if the image tiles doesn't have chinese labels baked into them, the service can't support it. In this case ArcGIS Online doesn't have any chinese tile services you can use, and it seems neither does Bing on their open imageservice, so you are basically out of luck. You will have to roll your own tile server using ArcGIS Server and your own data.