Here is a written example of differences between the Cantonese language and the Modern Standard Chinese (Mandarin) language when written in Chinese characters. I've seen many examples of signs or other written language in public places in Hong Kong that are incomprehensible to literate speakers of Mandarin, and some examples of written Taiwanese in Taiwan that are incomprehensible to people from anywhere else. How yo…
https://zh.wikipedia.org/zh-hk/%E7%B2%A4%E8%AF%AD#.E4.BF.9D....
In your example:
> 第一及第二人稱用「我」、「你」,與官話相同,但粵語音「我」(ngo5)更保留了中古漢語唐音(*ngɑ̌ )之疑母(ng-)。第三人稱不用「他」,而是「渠」(俗寫「佢」;東漢《孔雀東南飛》:「雖與府吏要,渠會永無緣」),跟吳語一樣。複數人稱不用「們」,而是上溯至端系的同源形式 [taʔ] 或 [ti](現代粵語寫作「哋」,本字為「等」,見聖公會的公禱書)。粵語用「係」而不用「是」來代表正面答覆,「係」是明清兩代常用字。這些字眼在中共接管政權初期,仍然有作書面語用,例如李儼《中算史論叢》第一冊 (1955年版本,第210頁) 提及18世紀數學家 Issac Wolfram 時,就寫為「渠係荷蘭礮隊副隊長」。
TL:DR Cantonese is Middle Age "Mandarin" in disguise, plus some Ancient Vietnamese dialects.