The phrase "are mostly concentrating" is correct and usable in written English.
It can be used when describing a group of people who are primarily focused on a particular task or activity.
Example: "The students are mostly concentrating on their final exams as the semester comes to an end."
Alternatives: "are primarily focused" or "are mainly concentrating".
While opposition groups are mostly concentrating on ending the brutish rule of President Bashar al-Assad, they are also positioning themselves for the longer-term question of who will rule in a post-Assad era.
That is one reason the agents are mostly concentrating on bottlenecks where the wood must be transported, catching loggers coming in and out of Alta Floresta, a city of about 50,000 people in northern Mato Grosso.
In contrast to the rival companies, which are mostly concentrating on providing ways to deliver the internet to mobile-phone towers or Wi-Fi relays on the ground, Loon balloons may be used as flying base stations, capable of talking directly to mobile devices on the ground.
In Tehran, the homeless are mostly concentrated in the Imamzade Abdollah neighbourhood in west Tehran, Shoush and Harandi, and parts of Azadegan Expressway in south Tehran.
Sidamo, also spelled Sidama, any of the Cushitic-speaking peoples of southwestern Ethiopia who are not Oromo; they are mostly concentrated in the Omo River and Rift Valley regions.
Cones are mostly concentrated within the central retina (macula), which contains the fovea (depression in the retina), where no rods are present.
One major question involves the presence in Japan of 47,000 American troops, which are mostly concentrated in the southern province of Okinawa, where they have long drawn strong local opposition.
The meridional membrane and bending signal components are mostly concentrated on the high-curvature areas, while the longitudinal bending component is mostly concentrated on the relatively flat areas.
