The language of the ancient kingdom of Baekje (18 BCE – 660 CE), one of the Three Kingdoms of Korea, is sparsely attested; indeed, it is not clear that what material exists is from the same language.
Since Baekje was established by immigrants from Goguryeo (the Buyeo-Baekje / Puyo-Paekche), it is presumed that they spoke the Goguryeo language, and several attested words support this idea; however, even if that is true, it is not known which language the indigenous Samhan people (Han-Baekje) spoke, or if the attested material may be a mix of Goguryeo and Samhan. The Gaya confederacy, however, was founded by one of the Samhan tribes, so it's possible that the old Baekje language was related to Gaya; based on toponymic evidence, it may be then that Buyeo-Baekje was related to Korean, and Han-Baekje, despite the name, to Japanese.
Baekje or Paekche (Hangul: 백제; hanja: 百濟, Korean pronunciation: [pɛktɕ͈e]) (18 BC – 660 AD) was a kingdom located in southwest Korea. It was one of the Three Kingdoms of Korea, together with Goguryeo and Silla.
Baekje was founded by Onjo, the third son of Goguryeo's founder Jumong and So Seo-no, at Wiryeseong (present-day southern Seoul). Baekje, like Goguryeo, claimed to succeed Buyeo, a state established in present-day Manchuria around the time of Gojoseon's fall.
Baekje alternately battled and allied with Goguryeo and Silla as the three kingdoms expanded control over the peninsula. At its peak in the 4th century, Baekje controlled most of the western Korean peninsula, as far north as Pyongyang, and may have even held territories in China, such as in Liaoxi, though this is controversial. It became a significant regional sea power, with political and trade relations with China and Japan.
In 660, it was defeated by an alliance of Silla and Chinese Tang Dynasty, submitting to Unified Silla.