Characters that modify letters are included in the Minecraft Font only when they actually modify letters, not when they don't modify anything.

Affected characters are: (List may be incomplete)
1. Umlaut (¨)
2. Circumflex (ˆ)
3. Tilde (˜)
4. Double acute accent (˝)
5. Acute (´)
6. Wedge (ˇ)

But when you type some modified letters, (for example: Ï, e̋, č, é ETC) they are in the normal font.