Who is responsible for this task?

I do not think the translator has done a good job as I am finding it hard to understand the situation you are trying to describe. 

Remember back when you studied for your PSM I & II exams, as that should allow you to come up with the answer you are looking for. 

From the scrum guide: 

The fundamental unit of Scrum is a small team of people, a Scrum Team. The Scrum Team consists of one Scrum Master, one Product Owner, and Developers. Within a Scrum Team, there are no sub-teams or hierarchies. It is a cohesive unit of professionals focused on one objective at a time, the Product Goal.

Scrum Teams are cross-functional, meaning the members have all the skills necessary to create value each Sprint. They are also self-managing, meaning they internally decide who does what, when, and how.

The Scrum Team is responsible for all product-related activities from stakeholder collaboration, verification, maintenance, operation, experimentation, research and development, and anything else that might be required. They are structured and empowered by the organization to manage their own work. Working in Sprints at a sustainable pace improves the Scrum Team’s focus and consistency.

 

The specific skills needed by the Developers are often broad and will vary with the domain of work. However, the Developers are always accountable for:

● Creating a plan for the Sprint, the Sprint Backlog;

● Instilling quality by adhering to a Definition of Done;

● Adapting their plan each day toward the Sprint Goal; and,

● Holding each other accountable as professionals.

 

The Product Owner is accountable for maximizing the value of the product resulting from the work of the Scrum Team. How this is done may vary widely across organizations, Scrum Teams, and individuals.

The Product Owner is also accountable for effective Product Backlog management, which includes:

● Developing and explicitly communicating the Product Goal;

● Creating and clearly communicating Product Backlog items;

● Ordering Product Backlog items; and,

● Ensuring that the Product Backlog is transparent, visible and understood.

The Product Owner may do the above work or may delegate the responsibility to others. Regardless, the Product Owner remains accountable.

 

The Scrum Master is accountable for establishing Scrum as defined in the Scrum Guide. They do this by helping everyone understand Scrum theory and practice, both within the Scrum Team and the organization.

The Scrum Master is accountable for the Scrum Team’s effectiveness. They do this by enabling the Scrum Team to improve its practices, within the Scrum framework.

maybe the translator did not do a good job. the problem was when the dev-team tells about an issue, but the issue is for those who are in contact for example with the customers, why do the people who are in contact with the customers dont understand that they have to fulfill this task? why do they think, that just because dev-team talks about an issue automatically they have to clarify it. scrum tells us that the one nearest to the problem and most suitable for it has to do it. right?

If resolution of the issue is necessary for the increment to be usable, whoever does the work to resolve it is effectively a Developer.

There is nothing in the Scrum Guide that says only the Product Owner can communicate with the stakeholders.  In fact there are places where it mentions the Scrum Team and stakeholders communicating and cooperating.  So why can't the developers, as part of the Scrum Team, ask the stakeholders the question directly? 

I'm going to point out something from the Scrum Guide that you, as Scrum Master, have responsibility to fulfill.  This comes from the section of the Scrum Guide that explains the way that the Scrum Master serves the organization (emphasis added by me)

The Scrum Master serves the organization in several ways, including:

• Leading, training, and coaching the organization in its Scrum adoption;

• Planning and advising Scrum implementations within the organization;

• Helping employees and stakeholders understand and enact an empirical approach for complex work; and,

• Removing barriers between stakeholders and Scrum Teams.

This seems like a perfect opportunity for you to do the two things I bolded.  A Product Owner is an employee and you seem to have a barrier between stakeholders and a portion of the Scrum Team.  There also seems to be some confusion on the part of the Scrum Team about the responsibilities they have, the level of communication and cooperation needed, and ownership of the outcome.