Hi Maciek,
Things like that come from files in the /Data/MessageTemplates folder.
The files are named with a language code prefix, and additional languages can be supported by copying the english files and renaming them for the new language and then translate the content.
It will automatically use the file for the language the site is configured for but if the file for that language does not exist it will fall back to the English version of the file.
The specific file you are asking about is:
/MessageTemplates/en-US-NewsletterVerificationEmailMessage.config
For more info see also Translating mojoPortal.
If you translate the file you can email it to me at joe dot audette at g mail dotcom and I will include it in mojoPortal.
Best,
Joe