But make it clear to them when they do this that there is a cost. "We estimated building the profile page based on doing it in one language; we can certainly internationalise it now, but it will take quite a bit longer". It's then up to them to decide whether the cost is worth the benefit.
As long as you are working as well and efficiently as you can on the work you have been given, you can feel satisfied with yourself.
Unless of course you're being judged for schedule slips caused by your product owner, in which case you're screwed, and you should quit and find a job somewhere less awful.
If you are employed and a product owner is required to give you your workload, you are kinda of stuck. The best you can do is illustrate (over time) that you are unable to get things done because of changing priorities.
Put together a collaborative document where you write what you understand to be your tasks. When a task changes or a new one is added, add a new entry with a date/time. Invite him to see it and ask him to bookmark it because you want to be clear on what you should be working on.
This is a good thing to have around review time as well, because you may be confronted with 'It doesn't seem like you are very productive'.
Also if it is possible that your priorities are not changing as much as you thought, this is a good exercise for yourself as well.
Use specific examples (so everyone can learn) and avoid making about people by focusing on the jobs to be done.
If you're not comfortable bringing this up to boss directly, shoot through a Sr dev first (who may coach you around the issues / validate your concerns), or, Sr dev may speak up when needed to support your cause should you bring it to the group/boss.