1. German, Dutch, Scandinavian and Czech feminine form of Johan, a continental variant of John, ultimately from Hebrew "God has been gracious". Hence it is a variant of Jane [q.v.]. Jonna is a Danish derivative of Johanna.
2. English, Scottish and Irish. Johanna is one of a number of spelling variations of Joanna that occur in the 1841 Census of Aberdeenshire. Joanna and Johanna are Latin forms of Joan, used in old documents with appropriate Latin endings.
A correspondent [MS] has noted that Johanna was a very popular name in Ireland, at least until the end of the 19th century, and is still in use today. The same corresondent [MS] asserts Johanna is the Anglicized form of the Irish name Siobhan.
3. Johanna is also the Latin form for Sian, the Welsh form of Jane.