As the article notes, the sentence "This sentence is written in English" is well understood, and true. "This sentence is written in French" is also well understood and false.
Not sure what the difference between the language of a sentence and the truthiness of a sentence is. Maybe it has to do with the fact that the language refers to syntactical features that can be determined at 'compile' time, while truthiness refers to a semantic quality feature that can only be determined at 'runtime'. (See en.wikipedia.org/wiki/colorless_green_ideas_sleep_furiously for at the very least a funny canonical example of a syntactically valid, but semantically nonsensical sentence.)
The fact that the language function can also be mapped to reduced parts of the sentence, might also be relevant, you can deduce that "This-" and "This sentence is-" are English, while you cannot partially compute that "This sentence is-" is true. A single negation bit being enough to change the result of the operation.
What does "This" in the above sentence refer to? It refers to the expression of an idea, how the idea is written, in what language. It does not refer to the idea, or the meaning of the sentence, what we think of as the "sentence" (vs. its encoding). Therefore I would say it is not self-referential sentence, or if it is in a very different way than "This" in "This sentence is false".