A protocol is necessary for systematic reviews because it reduces the risk of bias, increases transparency, ensures uniformity amongst the research team, and is ultimately what makes the review systematic.
The protocol includes inclusion and exclusion criteria, eligibility, search strategy, and how you'll assess the studies.
Note that to prevent bias you will need to include studies in other languages. Include in your protocol how you will find and translate these studies.
Your search strategy must be documented: where you are searching and what you are searching for (keywords, synonyms, kinds of studies).
Once you have a protocol, register it. One place you can do that is PROSPERO. In addition to increasing transparency, registering your protocol will let others know what you're researching, so they don't duplicate your work.