German and English, Latinized from Old German "bright". Used in England in Saxon times and reintroduced from Germany in the 19th century. It is also a diminutive for some German names containing the element -berht-, "famous".
Bertha occurs, rarely, in the 1841 Census of Aberdeenshire but slightly more frequently in 1881.
A correspondent notes the use of Bertha as an unusual pet name for Elizabeth.