How would you prioritize retirement contributions while living abroad (US citizen)?

I may be able to contribute to a 401(k) from a previous employer

Unlikely. 401(k) contributions come out of salaries, so if you no longer work for that employer you won't be able to continue contributing.

Given that, does it still make sense to contribute to a Roth if I’m not getting the tax-free growth benefit due to zero taxable income?

If all of your earned income is excluded then you cannot contribute to IRA either (neither Roth nor Traditional) since you have to have earned income to be at least the contribution amount.

I’m also unsure how foreign pension contributions factor into US tax or long-term retirement strategy

You'll need to talk to a US-licensed tax advisor (EA/CPA licensed in the US). You may end up liable for taxes on the contributions and the gains within the plan, and it may be subject to PFIC rules. You'll need a tax professional analyze the specific arrangement you have.

Given that, does it still make sense to contribute to a Roth if I’m not getting the tax-free growth benefit due to zero taxable income?

@littleadv gives a good answer to your general question (i.e. it seems you can't contribute to either right now), but I think you have this backwards. If you had a zero marginal tax rate, and were otherwise eligible to contribute to a Roth, then you absolutely would want to contribute to a Roth. You would pay no tax now and would get tax-free growth in the future.

It would NOT make sense to contribute to a Traditional IRA in this scenario because you would not get any tax benefit now, and the earnings would be taxed as earned income when you withdraw it.

I don't disagree with other answers, which focus on US tax accounts. But I'm going to mention the alternative which is to use the retirement savings accounts in the country you are working in.

If you are only going to be working in this country for one or two years... then your best bet is very likely not to contribute to a fund that is specifically for retirement. Such funds assume you are going to be keeping your funds there for many years. However you may want to keep putting money aside for your eventual retirement. In this case use whatever local tools there are for getting tax-free growth on money without long term commitment - such as an ISA in the UK or a TFSA in Canada. Then when you return to the US you can take that money out and put it in an American retirement account.

Alternatively, it may be a good idea to pause your retirement savings for a couple of years and use the money to take advantage of your location. Go and visit countries in Europe or Africa while you don't have to fly the Atlantic to reach them. The savings on doing it then, instead of waiting 'til later and adding the cost of airfare, are likely to be much higher than the tax savings you get from tax investment accounts. Not everything is about money.

If you are going to be working in Europe for several years, look at contributing to retirement investments in the local country. But get the advice of a local financial advisor, as there can be catches in when tax is charged.