The first letter of a sentence in Pinyin is to be capitalized. Does this remain true if the sentence starts with an arabic numeral? For instance, Google Translate turns "23万人" into "23 Wàn rén" instead of "23 wàn rén", as one may expect from European languages.
This may be hard to answer definitively. (It might be documented in the ISO 7098:2015, but it's behind a paywall.)
In any case, it would be natural to transliterate a sentence beginning:
25 wàn rén...
We certainly know sentences start with capital letters:
- Chūntiān láile.（春天来了。）
- Wǒ ài wǒ de jiāxiāng.（我爱我的家乡。）
(Basic Rules of the Chinese Phonetic Alphabet Orthography, 2012. Source.)
In the given example, the sentence starts with 2 (not 万).
It also seems reasonable to believe that pinyin follows English capitalization, like it does for other forms of capitalization (start of a sentence, people's names, etc.).
Some related observations...
It is possible to use Arabic numerals in pinyin, the above document contains this example:
along with examples where the pinyin corresponds to what is pronounced:
èr fēn zhī yī（二分之一）
èr líng líng bā nián（二〇〇八年）
I didn't find a definitive example where a numeral in Chinese was transliterated to its pronunciation in pinyin.
And there's also examples of place names where Chinese characters are converted to numbers in pinyin:
二马路 (2 Mǎlù)
三环路 (3 Huánlù)
(China Place Name Hanyu Pinyin Letter Spelling Rules (Hanyu Place Name Part), 1984. Source.)