Wow I don't know how but I managed to do it with the strings :) The custom field didnt accept php code so I made some changes to index.php and the magic happened. For those who are wondering how to use strings here how I managed to do it:
in functions.php I wrote just after the opening <?php tag: pll_register_string('intro_content','Text here','SPA');
It doesn't matter what you will type in the brackets actually. It is used just to find it in the String translation panel in Polylang options.
intro_content is the name, Text here is the string to be pulled out and SPA is my theme.
The string should be on the Polylang settings page now. Go and translate it for any added language.
Then in the custom field which you want to be translated just type <?php pll_e('Text here'); ?> and it will pull out the translation you made in the popylang settings page.
For my case the custom field don't accept php codes so I went to index.php ( where the custom field appear;) and replaced the php code there with the above one.
And this is my noob guide :) I know it is funny for the developers who understand perfectly the plugin's guide but after all not everyone who visits this forum is a professionalist.