I adore the French form,
Berthe, "behrt." (As you can see, I'm a sucker for the "behr" sound.) But the English form is too similar to the word "birth" for me.
That said, meeting a young
Bertha would absolutely make my day. I love seeing old names revived the same way I love to see old buildings brought back to life.
I don't see
Bertha making a come-back. But I don't see that as a reason not to use it. Start a trend. If she doesn't like her name, she could go by the oh so chic nn of Bear. :-)