How do you get buy-in from regional teams for a product launch when they don't view the product that you're launching as relevant for their market?

I love this question. Instead of trying to convince them, I would listen to their concerns and try to alter the product to make it relevant, especially if the market has a strong track record of being right. If you can’t make the feature requirements in time to do so, cut the market from the launch. There’s no need to enter a market (translation resourcing, training etc) if you don't have stakeholder buy-in. You will have more time to focus on other markets with more opportunity.

Also, nothing will make a market want a product more than seeing it be successful somewhere else.

This question is very similar to this other one https://sharebird.com/ama/zendesk-director-of-product-marketing-teresa-haun-on-product-launches?answer=mZxqPwjEr5 so sharing my answer from there here too:

At Zendesk, the vast majority of our functionality is global so almost every launch is a global launch. We determine the general approach and strategy that we think should work across all regions, but then partner with regional teams to understand how they may want to adjust specifically for their areas. Our regional teams localize pretty much everything, including campaigns and assets, so they’re determining what’s relevant for their markets and what isn’t. There are definitely times where they deem certain functionality isn’t relevant for their market. For example, one of the products my team owns only covered limited languages for a long time, so our APAC region in particular didn’t find much benefit from it. To address this, we worked with the APAC team to find ways to still promote the product where it made sense. For example, splitting their audience further to segments that did use the supported languages and therefore would still find that product relevant. We also would collaborate on how to address those gaps for the other segments through workarounds where we had them.

I always start with Why. Why don't the regional teams find that product valuable? Who exactly and what data (quant/qual) do they have to support it isn't valuable? Start with asking more questions - then present defensible proof points (from customer's who've adopted/expressed interest in the product) via case studies, utilization data, rev gen, etc - to show the value. Customers are the ultimate aligner, use their stories to get buy in where doubt has been cast. 

Putting on my PMM hat, I think it really depends. 

A few things to consider first:

(1) is the product officially GA in their market?

(2) how much "add on" stuff does the region need - whether it's customers configuring or ISVs/SIs filling the gap, to meet their needs? 

(3) have things like localization been factored in - e.g. if your product can only support US English content - how is it going to work in France or Japan? 

(4) try to understand why the sales team is thinking it's not relevant - if it's one of the above, you need to have clear answers and empathy when approaching it. If it's not, 

Assuming those basics are covered, approach it with empathy and put yourselves in their shoes. This will help you go into the conversation more prepared to think about all the reasons why they maybe objecting and have answers for them Or maybe you'll they were right :-)