Just keep in mind that badly edited .string files can cause malfunctions to the application, so check twice before editing and twice before saving.
Remember that the syntax of every line of localizable.strings and similar is

label (or "original string") = "string";

don't edit the label (or string) at the left of the equal sign and remember of the semicolon at the end of the line; also remember to reproduce in your modified string every occurence of the %@ token: it's a placeholder that the program will replace with an actual string (e.g: "Session started at @%" will be actually translated at runtime with something like "Session started at 10:22:15"); you can change its position, but cannot delete it.

(%@ are not the only tokens you can find: as a rule of thumb if you see "strange" @ sequences, just mantain them in your new string)

And also ^0, ^1 etc sometimes work.