So I guess Kashubian is a serverly endangered language because everybody who is able to speak Kashubian speaks Polish as well?
Yes, but there is more to it as well.
According to some sources, Kashubian is considered seriously endangered; spoken in Poland by 53,000 speakers (Polish 2002 Census),
A Kashubian born traveller to Kashubia observes:
..it transpires that few aficionados of the Kashubian culture speak the language, and the language is marginally present only in small, rural communities. Outside these communities, it is extremely rare to hear Kashubian spoken.
The estimates of the Kashubian speaking people significantly vary. For example this source, academia.pan.pl/pdfen/lifeforce_zieniuk.pdf estimates number of Kashubs is at 300,000-500,000, while the number of those who use Kashubian in speaking - at 150,000-300,000.
As many other sources stress, including the last two, Polish has functioned among the Kashubs as a language of literature, linguistic communication on an intellectual level, and education, while their native language has been used within the family and in local oral communication.
There is also serious fragmentation of Kashubian into geographical-based dialects, which cause problems for communication amongst the Kashubs themselves, as speakers from the southern dialect-area have difficulty in understanding those from the northern one.
It is well understood that for Kashubian ethnolect (this is a compromise word between "language" and "dialect") to survive a literary form of the language should be actively developed.
Certain religious texts (books and manuscripts) from the 16th and 17th century have been recognized as literary monuments of the Kashubian language, although they are in fact written in the Polish language of that time and only "inlaid" with Kashubian vocabulary and grammatical forms.
Attempts to create a Kashubian literary language per se date back to the mid-19th century. The first to write in the Kashubian ethnolect was Florian Ceynowa, who considered Kashubian to be a separate language, with Polish as its "elder brother."
Attempts at establishing a literary standard for Kashubian have subsequently been repeated in the 19th and 20th centuries. Beginning in the 1970s, other efforts have also been made towards normalizing, unifying spelling and "intellectualizing" the Kashubian language. Literary output, encompassing sacral and secular texts by authors of all generations, has increased significantly since 1990 and has been actively promoted by such institutions as the Kashubian-Pomeranian Association, the Kashubian Institute (Instytut Kaszubski), as well as media sources and educational institutions.
All of those things are also nicely described in details in Polish wikipedia,, pl.wikipedia.org/wiki/Język_kaszubski#cite_note-ele-2
However, the above sources do not give any proper credit to Aleksander Majkowski - a pre-war activist of Kashubian culture, a developer of Kashubian grammar, and an author of "Life and adventures of Remus" (Pol.: Życie i przygody Remusa, Kasz.: Żëcé i przigòdë Remùsa), a Kashubian epos; a novel considered the greatest example of Kashubian literature. I read, with difficulties of course, few stories from that book (available on line) and I liked them. Nowadays, "Remus" is available in various forms (old and modified spelling), including audiobooks.
One more thing: Originally, Kashubian language used to recorded with Polish alphabet (eg Hieronim Derdowski), but Polish did not provide full phonetic features of Kashubian. Here is for example, a famous "Marsz kaszubski" (Kashubian March) written and transcribed by Hieronim Derdowski, based on Polish national anthem:
Tom, gdzie Wisła do Krakowa
W Polscie morze płynie,
Polsko wiara, Polsko młewa,
Nigde nie zadzinie.
Nigde do zgube
Nie przyndu Kaszebe,
Marsz, marsz za wrodziem,
Me trzemume z Bodziem.