Adding a dictionary in different language and enabling spellchecking for that language too? I always install the system with English and have no idea how to add a Spanish dictionary and spellchecking on my mac, how can it be done?
You set the language for spellcheck in system preferences > keyboard > text > spelling, and at the app level in Edit > Spelling and Grammar > Show Spelling and Grammar.

Some non-Apple apps, like MS Office, have their own internal settings for this function.

You set your preferred reference dictionaries in Applications > Dictionary > Preferences.