Go to Settings in Mail.
Choose the tab Signatures in the Settings window.
Choose the email account you want to create a signature for, click the plus sign. Give the signature a descriptive name (e.g. UiO or English), and write in the wanted signature. Note that you can have more than one signature for each mail account.
Choose the signature that you want to be the default one for your outgoing mails.
Consider whether you want to activate the option Place signature above quoted text.
Insert signature in mail
When you send a mail, you have a menu for Signatures in the outgoing mail. Here you can select the desired signature.
The starting point is that signatures are not added automatically (None) and that you must actively choose a signature before sending the mail.
The next time you send a mail, Apple Mail will remember your last chosen signature.