I have a similar problem.
- I have downloaded and installed the Turkish language zip. The Email line and SUBSCRIBE button is translated but not the rest (see image 01).
- Can you please give a more step-by-step instruction on how we can do what you advise below (ie; where do I find the placeholder, how do I name it, how do I write the line to override the fields "First Name" and "Last Name"

?
- How do I actually remove the field names from above the entry lines and only leave the texts in the entry fields (see image 02)
,
You can name the placeholder com_blabla_name in mailchimp. And then you need to create a language override for com_blabla_name in joomla and this will be used.
Regards,
Daniel