How can I write emails in another language, with a spell check for that language? Is there a language dictionary to opt for?
If "Automatic by Language" does not give adequate results, you can set the dictionary to a specific language in Edit > Spelling and Grammar > Show Spelling and Grammar.
I do it all the time and yes the spell check is set to automatically detect.
There are few settings you have to do in syst preferences -keyboard like this.
If you need a different keyboard layout (characters) then set it up again in system preferences by adding that language so you can easy switch by clicking on the icon in the menu bar.
Not all language spelling are installed by default, so if yours is not a part of:
you can add it following this instructions: https://apple.stackexchange.com/a/11842/46541