This is a warning not to brag about what you expect to happen tomorrow. This can be stated clearly. AT: "Do not speak proudly about your plans for tomorrow" (See: [[rc://en/ta/man/translate/figs-explicit]])
Something happening on a certain day is spoken of as if the day were to bring that event. AT: "what will happen on a day" or "what will happen tomorrow" (See: [[rc://en/ta/man/translate/figs-metaphor]])
The words "let praise you" are understood from the first phrase. They can be repeated. Here a person is represented by his "mouth" and "lips" because those are the parts of the body used to speak. AT: "and do not let your own mouth praise you ... and do not let your own lips praise you" or "and do not praise yourself ... and do not praise yourself" (See: [[rc://en/ta/man/translate/figs-ellipsis]] and [[rc://en/ta/man/translate/figs-synecdoche]])
The words "let praise you" are understood from the first phrase. The can be repeated here. AT: "let a stranger praise you" (See: [[rc://en/ta/man/translate/figs-ellipsis]])