A certain Eastern European country (東欧某国 Tōō Bōkoku?) refers to four distinct locations that appear in the series which were not specifically identified, making it unclear if the locations overlap. They are presented in chronological order below.
This is the first European country to appear. It is where Jonah joined Koko's Squad and participated in Jonah's first mission, protecting Koko when she drove down to Hewlett Harbour to recover a shipment of MiG-29 upgrades that were being held up by customs. The covert unit Voskhod 6 falls under this country's Ministry of Internal Affairs and tried to stop Koko, but was defeated en route by Jonah and Valmet.
Following the previous event, C. K. Kloshkin tried to take advantage of the situation by selling the air force of Example 215 Hind-Ds. It is implied that this country borders the first example as it was mentioned earlier that upgrading the MiG-29s would affect the balance of power in the region. Koko found out about Kloshkin's move from HCLI headquarters and mobilised to deal with it as her pay would be cut if the deal went through. She succeeded in foiling the deal through a combination of force and Tojo's negotiating skills. In the anime the air force officer who Tojo negotiates with is named Stanistaw Jesensrg. Neither of his names are real, possibly an attempt to hint at his origins without referring to a specific country, but inferring a Slavic country, which includes Poland.
This example borders Russia and contains oil reserves. Koko's Squad was delivering a MIM-104 Patriot launcher to its military when they stumbled into a border war between the country and Russia over the control of a local pipeline. Pollack was the local area commander and attempted to obtain a replacement radar unit from Koko on Curry's suggestion. However she and her squad were able to escape and after Pollack set up the Patriot launcher, Russian bombing was halted and the fighting stopped.
The CIA maintains a black site in this example where terror suspects are held and executed for refusing to cooperate. Hex and Cutthroat were based here. In the manga the black site is at a remote facility in the mountains. However in the anime it is in a more urban location and is depicted as as a series of warehouses stated to be in Eastern Europe. It is not clear if the country is any of the previous examples or is a new one.