Figures in foreign language

I suggest you reference the source and add a note that you did the translation for the figure you show in your thesis.

A common approach is to show the figure in its original form, in much the same way as you would quote someone verbatim. Then, you would provide a translation of the important pieces in the caption. My recommendation would be to leave the figure itself unaltered since that allows you to preserve its authenticity and historical context.

In the end, this is the wrong forum to ask the question. The only person who can give you definitive advice on how to deal with the situation are the people who will need to approve your thesis. The first person to ask, therefore, would be your thesis adviser.