I have to write emails in many languages and it's always a pain to switch the language for spell-checking in KMail or Kontact. Is there an easy way?



Well, indeed, for people who are multilingual, or want to believe they are, working with spell-checking is always a bit of a trick!
You can set the automatic spell-checking (Figure 1). It's a very convenient feature that will highlight all your spelling errors, but the default KMail window does not offer a choice for languages! It's a pain. If your default language is set to French, any email in English will turn into a line of red!
Well, KDE is famous to have many options. People often complain about this, it is supposedly too complicated for newbies. I personally disagree. Those options are the difference between KDE and a shitty interface like in the windows world. Obviously, there is one for our little problem!
Go into the view menu and you'll see a dictionary option (Figure 2): that is simply what you're looking for! Once it is set, you will have a new line in the composer window with a choice of many languages (Figure 3)...
And finally, you'll be able to send all those emails with a minimal number of spelling errors!