He tells you this after visiting and / or donating to the museum many times, as with many other Special Characters in the game. (nookipedia.com)
You can also type a variety of accented letters and other special characters with Option key shortcuts. (howtogeek.com)
The layout is excellent, with essential punctuation keys available on the initial letters screen, and a good selection of other special characters available at a touch of the «123sym» button. (trustedreviews.com)