>How can I avoid Outlook to translate in English some words I enter in the body as soon as I move to another word? This is getting anyoing as sometimes it produces some errors that I did type ok in the first place.

Most probably you have enabled Word as the default editor. Try switching it off.
Select
. Tools/Options (menu)
. Mail Format (tab)
. uncheck the two boxes in the first section
. Use Microsoft Office Word...

Hope this helps,