This should appear on the advanced search panel, maybe as an additional expandable section.
only available for exact match searches
should work similar to search-and-replace in a normal text editor
"Find Next", "Replace", "Replace All", "Replace All" operations available
Find Next should select the next translation that includes the search text, and select or highlight the character range where the search text is found.
Find Next should stay on the same translation if there is a later occurrence of the search text in the translation.
Replace should substitute the replacement in the highlighted range, then perform Find Next
Replace All could apply to all matches in a phrase, in a document or in the selected set of documents. How/whether these options are presented should be figured out with some design work. Note: this feature is a replacement for project-wide search and replace in the GWT editor, so there must be a way to do a replacement in all the documents of a project with a small set of actions