Czesc Marcin,

Because you have to translate template and extension(s) language file, I mean phrases from it.

Start from template (at the bottom): language\en-GB\en-GB.tpl_shaper_educon.ini

If you have/use components you have to do the same, for example, language\en-GB\en-GB.com_splms.ini

in that 2nd file you will find phrases from SP LMS component, yes "Apply now" is there.

COM_SPLMS_APPLY_NOW="Apply Now"
COM_SPLMS_BUY_NOW="Buy Now"

You can override phrases using Language Override (J! core feature) or by creating a new pl-PL translation file. https://www.joomshaper.com/documentation/joomla-extensions/sp-lms/translation-of-sp-lms

In WordPress it works the same, nothing new.

In Joomla 1.x-6.x Language that you can install in Lanugage section is only for Joomla core.

There is OLD trick that can help you find out:

1. Use FTP
2. Rename folder: language\en-GB (for short period of time)
3. Then instead of english phrases on front-end you will see BIG-LETTERS - that you can use to translate words in Joomla Language Override

Okay, thank you. I'll give it a try.