I really don’t know. I think the trick is to bring the Monzo ethos / culture to each market individually, rather than directly translating stuff - which might makes it seem easier for English-speaking nations (especially as the biggest one - the US - is huge).
But the trade-off is, Ireland aside, it’s really difficult. You need to set up a local company, meet regulatory requirements - basically spend lots of time and effort. Which is more difficult the more geographically remote you are. And then there’s the terminology (which is what @anon44204028 might have been getting at) - remembering to talk about a checking account for a US customer vs a current account in the UK…
I’d be tempted to say (after Ireland and maybe in parallel to the US?) a large European market that hasn’t really got that much (fintech) competition. Germany has N26, so maybe France or Spain? But those would require a heavy commitment to localisation. France probably has the advantage, because then you have a staging ground to go into Belgium, Switzerland, Luxembourg, (Canada!) etc… Unlike N26, I think the key would be going the extra mile and making sure you have a domestic IBAN for each country - France, especially is still a bit behind in accepting direct debits from non FR- IBANs…
(Off-topic, I wonder if, on reflection, a very visible indicator of the country at the beginning of the IBAN was a bad idea - they are often hard coded and therefore difficult to change…)