I'm wondering if anyone can provide the origins of the honorific "-ちゃん". It's a diminutive, and German has "-chen" as a diminutive suffix. Is that a coincidence?
What is the first recorded use of "-ちゃん"?
Yes, it's most likely a coincidence. We can see this by comparing several titles:
The formal さま, which is clearly Japanese, was shortened to さん, which is now the most common and general title, and is more or less unmarked. さん was further reduced to the hypocoristic ちゃん, which is also very common, though not quite as much; and there is also a variant ちゃま reduced from さま in similar fashion at a later date, which is the least common of the four.
The earliest cite for ちゃん in 精選版 日本国語大辞典 is 1813, and for ちゃま it is 1900-01. Although both are after Japan had first been exposed to German, I think the large majority of borrowings from the German language were from the Meiji era and later (1868-), following the opening of the international borders in 1853. To me it seems like the simplest explanation is that given by dictionaries, that ちゃん is derived from さん, and is not related to German chen.
German and Japanese similarities here are purely coincidental. Japanese -chan derives as likely baby-talk from regular suffix -san. Similarly, we have regular -sama (which itself is the source of -san) and baby-ish -chama. See also most any JA-JA dead-tree dictionary, or the Daijirin entry here (see the third entry down), clearly stating:
As for first recorded use, I couldn't find any listing of citations in my dictionaries. It's clearly been around since at least some time before Natsume Sōseki's 1906 novel Botchan.