apparently because of the era mulan was in she should've spoken cantonese (or at least not mandarin) so why is mandarin the default chinese now?
hoo boy, this is a major misconception that i’ve heard a lot, especially from Cantonese speakers, who somehow believe that all people in pre-1000 AD China would’ve been speaking Cantonese ahahaha. I think this is an urban myth from some pseudo-linguistics rumours making its rounds in Cantonese-speaking communities over the last couple generations, probably stemming from the fact that Cantonese phonology is more conservative than Mandarin in terms of our syllable-final consonants, and the preservation of the voiced/voiceless distinction in Middle Chinese by extending our tonal inventory. A lot of people think that that automatically makes Cantonese the “pure Chinese language”, which makes me cringe really hard tbh, bc that’s not how languages work LOL.
The problem with the Sinitic languages is that we have no exact way of knowing when and how people spoke back in the day, and the characters give no 100% explicit phonetic clues, so the best we can use as reference are the rime tables that some smart Chinese linguists compiled way back in the day. The most famous one is the book of Qieyun rime tables, which helped modern linguists re-construct Middle Chinese forms. Middle Chinese is said to generally have been spoken around the 6th-11th century, or somewhere around that range, and the Qieyun tables were published sometime in the 6th century, so they would’ve reflected an early Middle Chinese variety that functioned as a standard language in that time period. The problem is that outside of this standard language, people in China were already speaking their own Sinitic languages like they are today, some of which were already mutually unintelligible from one another. Unfortunately, there’s no way to track exactly what languages were spoken because all we have are the rime tables.
The early forms of the modern varieties that we know today as Mandarin, Cantonese, Hakka, Wu, etc all split off from Late Middle Chinese presumably sometime between the 12th and 13th century, some languages later than others ofc (except for Hokkien and the other Min varieties, who had a bit of a special development, but that’s a different story). Mulan (who btw was a legendary character; we’re not even 100% sure if she actually existed) apparently lived during the Southern and Northern Dynasty period, which lasted from 420–589, which would fall into the Middle Chinese period, more specifically in the Early-Mid Middle Chinese period, so she probably would not have spoken anything close to Cantonese OR Mandarin in her time.
Another problem is that Mulan is said to have been from the Northern Wei Dynasty (北魏), which would’ve taken up the area north of the Yangtze River. Cantonese was historically spoken in southern China, particularly in the general regions of modern-day Guangdong and Guangxi. Guangzhou was always a very successful port city, even 1500 years ago, and became a very important cultural center even in the Southern Song Dynasty (particularly in the 12-13th century). In this time period, an early form of Cantonese had already developed, and it was also around this time that it gained a literary level to read the Chinese Classics. Therefore, it seems highly highly highly improbable that Mulan was a Cantonese speaker, considering that her time period and general geographical location did not even come close to matching the timeline of Cantonese or any other modern Sinitic variety.
Also to answer your question about why Mandarin is the standard today: it comes from a place of political power. Nanjing and Beijing (lit. Southern Capital and Northern Capital) were always the place of political power, and in courts they would use a standard language so that there could be communication between officials from all over China, hence the formal name for Mandarin: 官話 (lit. ‘official speech’). And that continued throughout history, simply because Beijing was always the source of political power, and made decisions for the country. However, the implementation of Mandarin as the ‘national language’ in all parts of China was a relatively recent event (around the mid-1900′s), in comparison to the entire history of the Chinese languages. Before Mandarin was heavily implemented in almost all parts of China, most people were still speaking local varieties, which would have mainly consisted of developing forms of modern Sinitic varieties.
TL;DR - Nope, contrary to popular belief, Mulan most likely did not speak Cantonese, and neither did a lot of early famous Chinese figures like Confucius, Mencius, Laozi, etc.