Lao's mutual intelligibility with Thai and Isan, fellow Southwestern Tai languages, allows for effective intercommunication among their speakers, despite differences in script and regional variations.

Internationally, Lao is spoken among diaspora communities, especially in countries like the United States, France, and Australia, reflecting its global diasporic presence.

[7] Due to the influx of Han Chinese soldiers and settlers, the end of the Chinese occupation of Vietnam, the fall of Jiaozhi and turbulence associated with the decline and fall of the Tang dynasty led some of the Tai peoples speaking Southwestern Tai to flee into Southeast Asia, with the small-scale migration mainly taking place between the eighth and twelfth centuries.

For instance, the Proto-Southwestern Tai *mlɛːŋ has produced the expected Lao /m/ outcome maeng (ແມງ mèng, /mɛ́ːŋ/) and the expected Thai /l/ outcome laeng (แลง /lɛ̄ːŋ/), although this is only used in Royal Thai or restricted academic usage, with the common form malaeng (แมลง /máʔ.lɛ̄ːŋ/), actually an archaic variant.

Khmer loan words include body parts, urban living, tools, administration and local plants.

Scholars such as Khanittanan propose that the deep genetic and linguistic impact of the autochthonous Khmer and their language indicates that the earliest days of Ayutthaya had a largely bilingual population.

[12][14] Indic languages also pushed Thai and Lao closer together, particularly Sanskrit and Pali loan words that they share.

Many Sanskrit words were adopted via the Khmer language, particularly concerning Indian concepts of astrology, astronomy, ritual, science, kingship, art, music, dance and mythology.

Fa Ngoum was a grandson of the ruler of Muang Xoua (RTGS Mueang Sawa), modern-day Louang Phrabang.

Lan Xang was religiously diverse, with most of the people practicing Tai folk religion albeit somewhat influenced by local Austroasiatic animism, as well as the Brahmanism and Mahayana Buddhism introduced via the Khmer and Theravada Buddhism which had been adopted and spread by the Mon people.

The spread of Theravada Buddhism spread literacy, as monks served as teachers, teaching reading and writing as well other basic skills to village boys, and the Tai Noi script was used for personal letters, record-keeping, and signage, as well as to record short stories and the klon (ກອນ /kɔ̀ːn/) poetry that were often incorporated into traditional folksongs.

This began to change when the golden age of Lao prosperity and cultural achievements under King Sourignavôngsa (ສຸຣິຍະວົງສາ /súʔ.līʔ.ɲāʔ wóŋ.sǎː/) (1637–1694) ended with a successional dispute, with his grandsons, with Siamese intervention, carving out their separate kingdoms in 1707.

The arid hinterlands, deforested and depopulated after a series of droughts, likely led to the collapse of the Khmer Empire, was only occupied by small groups of Austroasiatic peoples and scattered outposts of Lao mueang in the far north.

[citation needed] The bulk of the Lao, however, settled after 1778 when King Taksin, Siamese king during the Thonburi Period (1767–1782) conquered Champasak and Vientiane and raided Phuan areas for slaves, seizing the Emerald Buddha and Phra Bang (although the latter was eventually returned) and forcing some of the Lao across the river to settle in Isan.

[19] Although the kingdoms remained nominally autonomous, the Siamese demanded tribute and taxes, kept members of the respective royal houses as hostages to ensure loyalty and required the three Lao kings to come to the capital several times a year to hold an audience with the Siamese king.

When the kingdoms revolted, the Siamese armies retaliated by rounding up entire villages, tattooing them to mark them as slaves and forced to settle what is now Isan, forced to serve as soldiers or manpower in corvée projects to build roads, to grow food, build canals, or serve as domestics.

This greatly expanded the Lao population of Isan and caused assimilation of the local peoples into the mix, a process that is occurring on a smaller scale even now.

Temples built in what is now Isan still featured the Tai Noi script on its murals and although Siam would intervene in some matters, daily administration was still left to the remaining kings and various Lao princes that served as governors of the larger mueang.

In Isan, Vientiane Lao is the primary form of Isan spoken in the northern third of the region which was long settled since the days of Lan Xang and was ruled as part of the Kingdom of Vientiane, including most of Nong Khai, Nong Bua Lamphu, eastern Loei and portions of Saiyaphum and Bueng Kan. As a result of the Lao rebellion of 1826 the Tai Wiang (ໄທວຽງ), /tʰáj wíaŋ/), 'Vientiane people' of the city and surrounding parts of the kingdom, were rounded up by Siamese armies and forced to the right bank, greatly boosting the Lao population of what is now Isan.

The Tai Wiang strengthened numbers in the northern third, where Vientiane Lao was traditionally spoken, but were scattered across the Isan region overall, with heavier concentrations in Yasothon, Khon Kaen, and Hoi Et provinces.

[6] The dialect is not common in Isan, restricted to western portions of Loei and pockets of villages spread throughout Udon Thani provinces.

In the other northern Laotian provinces of Oudômxai, Houaphan, Louang Namtha and Phôngsali, native Lao speakers are a small minority in the major market towns but Northern Lao, highly influenced by the local languages, is spoken as the lingua franca across ethnic groups of the area.

Louang Phrabang remains the largest city in the northern region of Laos, serving as an important center of trade and communication with the surrounding areas.

There are also small pockets of speakers located in Steung Treng Province, Cambodia or Siang Taeng (ຊຽງແຕງ, /síaŋ tɛːŋ/), particularly near the Mekong River close to the Laotian border.

Combined with the somewhat faster manner of speaking and reduced tendency to soften consonants at the end of words, Southern Lao sounds very rough and harsh to speakers of other dialects.

Many of these features, such as the faster speaking pace and glottalization may be influences from Austroasiatic languages as most of the region was inhabited by the Khmer, Kuy and various other Austroasiatic peoples until the eighteenth century when the Lao began to settle and even now, Khmer speakers comprise half the population of Surin and roughly a quarter each of the populations of Sisaket and Buriram provinces.

[27] Specific dialectal words include don (ດອນ, /dɔːn/), 'riparian island', se (ເຊ xé, /sȅː/)) and many of the words used in Savannakhét that are more typical of Southern Lao such as mae thao (ແມ່ເຖົ້າ, /mɛ́ː tʰȁw/) as a respectful term for an 'old lady' (as opposed to Vientiane 'mother-in-law') and use pen sang (ເປັນສັງ, /pȅn sȁŋ/) instead of Vientiane pen yang (ເປັນຫຍັງ pén gnang, /pen ɲăŋ/), 'what's wrong?'.

Formal writing has a larger number of loanwords, especially Pali and Sanskrit, much as Latin and Greek have influenced European languages.

The complete inventory of Lao consonants is as shown in the table below:[37][38] All plosive sounds (besides the glottal stop /ʔ/) are unreleased in final position.

Special particle words serve the purpose of prepositions and verb tenses in lieu of conjugations and declensions.