Siouan languages
In the 19th century, Robert Latham suggested that the Siouan languages are related to the Caddoan and Iroquoian languages. In 1931, Louis Allen presented the first list of systematic correspondences between a set of 25 lexical items in Siouan and Iroquoian. In the 1960s and 1970s, Wallace Chafe further explored the link between Siouan and Caddoan languages. In the 1990s, Marianne Mithun compared the morphology and syntax of all the three families. At present, this Macro-Siouan hypothesis is not considered proved, and the similarities between the three families may instead be due to their protolanguages having been part of a sprachbund.
French language
French was the official language of the colony of French Indochina, comprising modern-day Vietnam, Laos, and Cambodia. It continues to be an administrative language in Laos and Cambodia, although its influence has waned in recent years. In colonial Vietnam, the elites primarily spoke French, while many servants who worked in French households spoke a French pidgin known as "Tây Bồi" (now extinct). After French rule ended, South Vietnam continued to use French in administration, education, and trade. Since the Fall of Saigon and the opening of a unified Vietnam's economy, French has gradually been effectively displaced as the main foreign language of choice by English. French nevertheless maintains its colonial legacy by being spoken as a second language by the elderly and elite populations and is presently being revived in higher education and continues to be a diplomatic language in Vietnam. All three countries are official members of the OIF.