I see this job as a foot in the door and a way for me to maybe later change job to something that is more interesting.
This is the key; a foot in the door is how it works, and the contacts/friendships you'll hopefully make there especially among other Scandinavian people (since it's a Nordic bank) will hopefully be very useful indeed for you in the future.
You do say that you might find you enjoy it (and presumably stay longer). It was very good to read that because it's how careers often work. An opportunity presents itself, you find that there are things that you like about it(you might even say "yes, this is it"), that then leads to another door opening if they find you reliable, positive, capable and (this is so so important) easy to get along with.
In a lot of sectors if you find a tight specialism (and one with a future) this is good. Excellent references do help a lot; something that helps even more is personal recommendations. Managers talk about excellent employees and recommend them not just in a reference, but often earlier in the recruitment process. I'm hiring now (for unusual and highly specialised jobs in an unusual place) and people who are recommended to me by people who value their reputation/friendship enough to only recommend good people are far more likely to get the work than people who come through adverts or an agency.
I didn't really think I'd like what I'm doing now; my plan was to stay for a few months and then move on to something specific I was waiting for in the UK, at what used to be The Royal Military College of Science; I took it because one aspect of the work is close to my specialism and I was interested in the particular location of the work. A large part of the role was outside my normal comfort zone, however things went well, my face fitted, and I was offered something rather specific (they created the post, I wrote the job description and specifications) and it is only likely to get better. This (if you're lucky, quick off the mark, don't 'hustle' and are both able and positive) is exactly how things can (and should) work out in the end. If they respect and like you (I think banks are similar although it's years since I worked at one), they'll help you.
Re. your real question, about whether the salary is enough to live on, the answer is yes. It isn't a fortune by any means, however the flat does make all the difference. Live like a local; it doesn't hurt. I do suggest you save as much as possible since it's important to have a cushion if you decide that the work (and the place) isn't for you.
And as I say, Łódż can be a very nice place to be. I wish you all the very best in the job.
I have businesses in PL and employ Polish people
You don't ''have businesses'', Adrian, you're a 20-something in and from America. BTW I speak only Polish at home and among friends and passed the state language tests nicely. That and of course having lived in PL for as long as you've been alive means I probably comprehend the nuances' rather better than you think. And really have owned (two) businesses there. Real ones.