Often there are several translation options for the same strings. We support upvoting so contributors decide which option is the best.
Begin by enabling Translation upvoting in project settings (it's turned off by default). Please note, it would automatically enable Reviewing for the project, as the reviewing should be used to override translation version rating.
Once both options are enabled, it is important to give some of the users reviewer access. Navigate to project's Contributors page and turn on reviewer switch for users who are allowed to override the voting by reviewing the translation.
With upvoting enabled, all versions of the translation (ever submitted by contributors) will be displayed in the inline suggestions dropdown once you click according translation. Users without re access will have the option to upvote or downvote each version or enter a new one. Number on the right side of the up/down vote arrows shows positive or negative balance of particular translation entry. Each user can give only one vote (up or down) for each of the translation versions (this applies to versions submitted by other users).
Translation version rating does affect the fact which version becomes "main" and will be included in project export, however if the translation is reviewed (approved) the rating changes would not affect "main" translation until the reviewer either removes reviewed flag or reviews another version.