There is a bug is Yahoo which causes the language displayed to change randomly
All my setting on Yahoo say to use English as the language. I use a VPN when traveling, and occasionally it will switch to a foreign site and then suddenly every thing on Yahoo is in a foreign language.
Of course I can manually select a VPN in an English speaking country, but your displays should match the customers settings, not randomly change. This is a Yahoo issue and it doesn't happen on Google.