>>>> and the use of the word [ja:] ("ja"). <<
Does that mean that you say ja for yes, like in California English?<<
We still have the words "yes" [jEs] and "yeah" [jE{] or [jE] in addition to "ja" [ja:]. But as for Californian English "ja" and "ja" here, Californian English "yah" (I really don't like calling it "ja", as the use of that spelling is primarily to signify that such is a non-English Germanic loanword) is just English "yeah" with historical [{] lowered to [a], whereas "ja" here is of a distinctly different origin, being a loanword from other Germanic languages (typically High German dialects or Norse languages, but note that Low German languages have also been spoken here).
One note though is that it appears that many people here who do use "ja" do not keep it distinct from "yeah" in their minds, and will insist that they do not use "ja" even when they may be using "ja" in the very same conversation. When called on it, they very often use "yeah" [jE{] instead. Consequently, the use of "ja" is most often noticed by people who do not natively use "ja" who come in contact with those who do, who in general very strongly notice the use of "ja".
Does that mean that you say ja for yes, like in California English?<<
We still have the words "yes" [jEs] and "yeah" [jE{] or [jE] in addition to "ja" [ja:]. But as for Californian English "ja" and "ja" here, Californian English "yah" (I really don't like calling it "ja", as the use of that spelling is primarily to signify that such is a non-English Germanic loanword) is just English "yeah" with historical [{] lowered to [a], whereas "ja" here is of a distinctly different origin, being a loanword from other Germanic languages (typically High German dialects or Norse languages, but note that Low German languages have also been spoken here).
One note though is that it appears that many people here who do use "ja" do not keep it distinct from "yeah" in their minds, and will insist that they do not use "ja" even when they may be using "ja" in the very same conversation. When called on it, they very often use "yeah" [jE{] instead. Consequently, the use of "ja" is most often noticed by people who do not natively use "ja" who come in contact with those who do, who in general very strongly notice the use of "ja".