I just received this note from a colleague:
I found a document on the Hong Kong Education Bureau's website that says: "Xiānggǎng de qíngkuàng shì yǐ Zhōngwén wéi mǔyǔ 香港的情況是以中文為母語" ("The situation in Hong Kong takes Chinese as the Mother Tongue").
Zhōngwén 中文 ("Chinese") is a rather curious, ambiguous, and imprecise term since it can essentially mean just about any kind of Chinese. I think using it to refer to a person's so-called mother tongue is especially dubious and sneaky.
For many reasons, I would have to agree with my colleague. First of all, I have always considered Zhōngwén 中文 ("Chinese") as a written language (wén 文), not a spoken language (huà 话, yǔ 语). Consequently, from the time I began studying Mandarin (in 1967), I always experienced cognitive dissonance when people asked me: "Nǐ huì bù huì jiǎng Zhōngwén 你会不会讲中文?" ("Can you speak written Chinese?").
Rather than being anyone's Mother Tongue, Zhōngwén 中文 ("Chinese") is the written language that everyone in China who aspires to literacy must learn. Among many others that could be cited, see these Language Log posts for some relevant observations on this matter, with particular reference to Cantonese:
Zhōngwén 中文 ("Chinese") is the written form of the national standard language (Pǔtōnghuà 普通话 / Guóyǔ 国语 / Huáyǔ 华语 [N.B.: these are all (huà 话 or yǔ 语, i.e., spoken language, but not native to any particular place) that replaces one's Mother Tongue. See "How to Forget Your Mother Tongue and Remember Your National Language"
In this connection, I would like to briefly consider Norwegian Bokmål, which is currently by far the most common written form of Norwegian. I'm not sure what the -mål part of the name means (the Wikipedia article seems to indicate that it means "tongue", but -- so far as I know -- it usually means "goal; aim; target; measure[ment]"). Regardless of what exactly -mål signifies, the Bok- part of the name means "book", hence written. May I ask those readers of this post who know Norwegian whether it would sound natural for someone to say "Do you speak Bokmål?" Ditto for all the other languages with which we deal; does it make sense, or is it customary, for someone to say that they speak "the written form of X language"?