Countries in Southeast Asia (2020-05-04)
Country
Etymology of the country name
Population
Ethnic groups
Languages
Religions
Brunei

Borneo (place name).
464,478
Malay 65.7%, Chinese 10.3%, other 24%
Malay (Bahasa Melayu) (official), English, Chinese dialects
Muslim (official) 78.8%, Christian 8.7%, Buddhist 7.8%, other (includes indigenous beliefs) 4.7%
Cambodia

Kampuchea, Kambujadeśa “land of Kambuja”, from an origin-myth.
16,926,984
Khmer 97.6%, Cham 1.2%, Chinese 0.1%, Vietnamese 0.1%, other 0.9%
Khmer (official) 96.3%, other 3.7%
Buddhist (official) 97.9%, Muslim 1.1%, Christian 0.5%, other 0.6%
Indonesia

Indos “India”+ nesos “islands” in Greek.
nusantara
267,026,366
Javanese 40.1%, Sundanese 15.5%, Malay 3.7%, Batak 3.6%, Madurese 3%, Betawi 2.9%, Minangkabau 2.7%, Buginese 2.7%, Bantenese 2%, Banjarese 1.7%, Balinese 1.7%, Acehnese 1.4%, Dayak 1.4%, Sasak 1.3%, Chinese 1.2%, other 15%
Bahasa Indonesia (official, modified form of Malay), English, Dutch, local dialects (of which the most widely spoken is Javanese)
note: more than 700 languages are used in Indonesia
Muslim 87.2%, Protestant 7%, Roman Catholic 2.9%, Hindu 1.7%, other 0.9% (includes Buddhist and Confucian), unspecified 0.4% 
Laos

Lao (ethnic group name), plural s in French.
7,447,396
Lao 53.2%, Khmou 11%, Hmong 9.2%, Phouthay 3.4%, Tai 3.1%, Makong 2.5%, Katong 2.2%, Lue 2%, Akha 1.8%, other 11.6%
note: the Laos Government officially recognizes 49 ethnic groups, but the total number of ethnic groups is estimated to be well over 200
Lao (official), French, English, various ethnic languages
Buddhist 64.7%, Christian 1.7%, none 31.4%, other/not stated 2.1%
Malaysia

Malay (ethnic gropu name) + sia, in Greek.
32,652,083
Bumiputera 62% (Malays and indigenous peoples, including Orang Asli, Dayak, Anak Negeri), Chinese 20.6%, Indian 6.2%, other 0.9%, non-citizens 10.3%
Bahasa Malaysia (official), English, Chinese (Cantonese, Mandarin, Hokkien, Hakka, Hainan, Foochow), Tamil, Telugu, Malayalam, Panjabi, Thai
note: Malaysia has 134 living languages - 112 indigenous languages and 22 non-indigenous languages; in East Malaysia, there are several indigenous languages; the most widely spoken are Iban and Kadazan
Muslim (official) 61.3%, Buddhist 19.8%, Christian 9.2%, Hindu 6.3%, Confucianism, Taoism, other traditional Chinese religions 1.3%, other 0.4%, none 0.8%, unspecified 1%
Myanmar

Bamar (ethnic group name), results in Burma and Myanmar.
56,590,071
Burman (Bamar) 68%, Shan 9%, Karen 7%, Rakhine 4%, Chinese 3%, Indian 2%, Mon 2%, other 5%
note: government recognizes 135 indigenous ethnic groups
Burmese (official)
note: minority ethnic groups use their own languages
Buddhist 87.9%, Christian 6.2%, Muslim 4.3%, Animist 0.8%, Hindu 0.5%, other 0.2%, none 0.1%
note: religion estimate is based on the 2014 national census, including an estimate for the non-enumerated population of Rakhine State, which is assumed to mainly affiliate with the Islamic faith; as of December 2019, Muslims probably make up less than 3% of Burma's total population due to the large outmigration of the Rohingya population since 2017
Philippines
Named after KIng Philip II of Spain (r. 1556-98).
109,180,815
Tagalog 24.4%, Bisaya/Binisaya 11.4%, Cebuano 9.9%, Ilocano 8.8%, Hiligaynon/Ilonggo 8.4%, Bikol/Bicol 6.8%, Waray 4%, other local ethnicity 26.1%, other foreign ethnicity .1%
unspecified Filipino (official; based on Tagalog) and English (official); eight major dialects - Tagalog, Cebuano, Ilocano, Hiligaynon or Ilonggo, Bicol, Waray, Pampango, and Pangasinan
Roman Catholic 80.6%, Protestant 8.2% (includes Philippine Council of Evangelical Churches 2.7%, National Council of Churches in the Philippines 1.2%, other Protestant 4.3%), other Christian 3.4%, Muslim 5.6%, tribal religions .2%, other 1.9%, none .1%
Singapore

Singa + pura < siṃha “lion” + pūra “city” in Sanskrit from an origin myth.
6,209,660
Chinese 74.3%, Malay 13.4%, Indian 9%, other 3.2%
note: individuals self-identify; the population is divided into four categories: Chinese, Malay (includes indigenous Malays and Indonesians), Indian (includes Indian, Pakistani, Bangladeshi, or Sri Lankan), and other ethnic groups (includes Eurasians, Caucasians, Japanese, Filipino, Vietnamese)
English (official) 36.9%, Mandarin (official) 34.9%, other Chinese dialects (includes Hokkien, Cantonese, Teochew, Hakka) 12.2%, Malay (official) 10.7%, Tamil (official) 3.3%, other 2% (2015 est.)
note: data represent language most frequently spoken at home
Buddhist 33.2%, Christian 18.8%, Muslim 14%, Taoist 10%, Hindu 5%, other 0.6%, none 18.5%
Thailand

Thai (ethnic group name, Siam) + land
68,977,400
Thai 97.5%, Burmese 1.3%, other 1.1%, unspecified <.1%
note: data represent population by nationality
Thai (official) only 90.7%, Thai and other languages 6.4%, only other languages (includes Malay, Burmese)
note: data represent population by language(s) spoken at home; English is a secondary language of the elite
Buddhist 94.6%, Muslim 4.3%, Christian 1%, other
上座仏教 Theravada Buddhism, Hinayana Buddhism, Mahayana Buddhism
Timor-Leste

Timur “East” in Malay. Leste “East” in Portugese.
1,383,723
Austronesian (Malayo-Polynesian) (includes Tetun, Mambai, Tokodede, Galoli, Kemak, Baikeno), Melanesian-Papuan (includes Bunak, Fataluku, Bakasai), small Chinese minority
Tetun Prasa 30.6%, Mambai 16.6%, Makasai 10.5%, Tetun Terik 6.1%, Baikenu 5.9%, Kemak 5.8%, Bunak 5.5%, Tokodede 4%, Fataluku 3.5%, Waima'a 1.8%, Galoli 1.4%, Naueti 1.4%, Idate 1.2%, Midiki 1.2%, other 4.5%
note: data represent population by mother tongue; Tetun and Portuguese are official languages; Indonesian and English are working languages; there are about 32 indigenous languages
Roman Catholic 97.6%, Protestant/Evangelical 2%, Muslim 0.2%, other 0.2%
Vietnam
Việt “越” (ethnic group name) + Nam “南, south” in Chinese. Official name used since 19th c. from an origin myth.
98,721,275
京 Kinh (Viet) 85.7%, Tay 1.9%, Thai 1.8%, Muong 1.5%, Khmer 1.5%, Mong 1.2%, Nung 1.1%, Hoa 1%, other 4.3%
note: 54 ethnic groups are recognized by the Vietnamese Government
Vietnamese (official), English (increasingly favored as a second language), some French, Chinese, and Khmer, mountain area languages (Mon-Khmer and Malayo-Polynesian)
Buddhist 7.9%, Catholic 6.6%, Hoa Hao 1.7%, Cao Dai 0.9%, Protestant 0.9%, Muslim 0.1%, none 81.8%