I live in a Dutch-speaking region, and my emails - which are usually in English - are often automatically "translated" from Dutch to English! The result is only that some words are changed, or their spelling is changed.
Only email from Dutch advertisers is originally in Dutch. That is also automatically translated to English. I have no objection to that, but I would prefer to lose that facility and have the English emails not interfered with.
Often a sign comes up telling me that the translation from Dutch to English has not been successful, but the words are still changed. That message has a "try again" option and an X (cancel) option. If I press the X option, the message simply comes up again.
I have searched Yahoo "help" with no success. There is no relevant heading.

What E-mail are you using? Yahoo? There should be settings somewhere on your mail page where you can turn off auto translation. Have you installed any translation software on your PC? That could be the culprit as well.

I had the same problem of my French friend's emails always auto-translating to English. In my mail, I clicked the "settings" button and selected the option to use the old version of Yahoo mail, and when it switched to that version, there was no auto-translation any more. I then switched back to the newer version, and the auto-translation had ceased there too.