Chinese
(1920s) — replaced Classical Chinese with Vernacular Chinese as the standard written language.
Mandarin was chosen at a committee from several Chinese dialects.
(1950s PRC) — reformed the script used to write the standard language by introducing Simplified Chinese characters (later adopted by Singapore and Malaysia, but Traditional Chinese characters remain in use in Taiwan, Hong Kong, Macau, and various overseas Chinese communities).
Czech (19th century) — The dictionary of Josef Jungmann contributed to the renewal of the vocabulary. In the 1840s the letter w became replaced by v.
Estonian (1910s/1920s) — reform movement led by Johannes Aavik and Johannes V. Veski renewed the vocabulary, borrowing a lot of roots from Finnish and other Uralic languages and even inventing some roots.
German (1901/02) — unified the spelling system nationwide (first in Germany, with later adoption by other Germanophone countries). Further reforms were enacted more recently, in the German spelling reform of 1996.
Greek (1970s/1980s) — while the written "pure" language, the katharevusa was full of Old Greek words, the spoken "popular" language, the dhimotiki was not. After the fall of the military rule, a law was promulgated, making the latter become the written language as well. For example, on Greek coins, the plural of the currency was drachmai (katharevusa form) before and became drachmes (dhimotiki form) after 1982.
Hebrew (1920s) — Modern Hebrew was created from Ancient Hebrew by simplification of the grammar (especially of the syntax) according to Indo-European models, coinage of new words from Hebrew roots based on European models, and simplification of pronunciation rules. Linguist Ghil'ad Zuckermann argues that Modern Hebrew, which he terms "Israeli", is a Semito-European hybrid, based not only on Hebrew but also on Yiddish and other languages spoken by revivalists.[1] Zuckermann therefore endorses the translation of the Hebrew Bible into what he calls "Israeli".[2]
Hungarian (late 18th and early 19th centuries) — more than ten thousand words were coined,[3] out of which several thousand are still actively used today (see also Ferenc Kazinczy).
Irish (1940s) — spelling system greatly simplified: Gaedheal became Gael, and Ó Séigheadh became Ó Sé.
Japanese (1946) — historical kana usage was replaced by modern kana usage, while the kanji system also transformed from Traditional Chinese characters to New Characters.
Norwegian (20th century) — as Norway became independent from Denmark (1814), Norwegian started to drift away from Danish. The reforms in 1907 and 1917 made Riksmål the written standard Norwegian, renamed Bokmål in 1929. Bokmål and the more vernacular Nynorsk were made closer by a reform in 1938. Today both language forms are spoken: on Norwegian coins, the name of the country is alternately Norge (Bokmål) and Noreg (Nynorsk).
Portuguese (20th century) — replaced a cumbersome traditional spelling system with a simplified one (asthma, for instance, became asma and phthysica became tísica).
Romanian (19th century) — replaced Cyrillic script with the Latin alphabet, deprecated hundreds of Slavic words in favour of Romance ones. Romanian has undergone spelling reforms in 1904, 1953, and, most recently, in 1993, with two minor ones in 1964 and 2005.
Somali (1970s) — modified Latin script developed by Somali linguist Shire Jama Ahmed for writing the Somali language; made compulsory in 1972 by the President of Somalia, General Mohamed Siad Barre. Also the vocabulary was renewed, a lot of new words became coined from existing Somali roots.
Turkish (1930s) — language and writing system were reformed starting in the 1920s, to the point that the older language is called by a different name, Ottoman Turkish. The Ottoman alphabet was based on the Arabic alphabet, which was replaced in 1928 by the new, Latin-based Turkish alphabet. Loanwords of Persian and Arabic origin were dropped in favor of native Turkish words or new coinages based on Turkic roots.
Vietnamese (20th century) — Classical Chinese lost official status in 1918, and the colonial schools instituted a "Franco-Vietnamese Curriculum" at this time. Vietnamese was taught using the Latin alphabet, and this form soon became dominent.
Let's make this a first on ACF...a FUN argument. 
