sentence-end-without-space

sentence-end-without-space is a variable defined in `paragraphs.el'.
Its value is

"。．？！"

• This variable is safe as a file local variable if its value
  satisfies the predicate `stringp'.

Documentation:
String of characters that end sentence without following spaces.

This value is used by the function `sentence-end' to construct the
regexp describing the end of a sentence, when the value of the variable
`sentence-end' is nil. See Info node `(elisp)Standard Regexps'.

You can customize this variable.