I have a big problem with writing mail in Polish because all diacritic symbols are achieved by right alt+letter and this interferes with Yahoo shortcuts.
Is there a more convenient way than editing the mail somewhere else?
I think the problem is not with Yahoo Mail Shortcut as it has no keyboard shortcut as right alt + letter (check Yahoo Mail keyboard Shortcuts), instead the problem is with your browser's keyboard shortcuts. So try disabling/configuring those or consider using a different browser.
For Google Chrome Shortcut Manager may be helpful.