Plan your project carefully with all your points of contact: software engineers, technical writers, vendors, test team, and target-country product marketing. The latter will set the release date, and, assuming that it is a feasible one giving the development schedule, this will be your objective. This is the date around which you must work.

The other constraints are:

Use Microsoft Project, or any other project management application, to assign time and resources for the various tasks. Include as much detail as you can. It is very important that you add buffer periods at various points, particularly toward the end, to cater for possible (and likely!) overruns.

While your vendor is likely not interested in the level of detail included in the project plan, the vendor will need to know the planned delivery schedule of the source bundles, and the milestones for delivery of the translated material.

A project plan is a plan. As events unfold, the plan will change. There will be many surprises. A software UI bundle that you expected to contain 500 strings will suddenly contain 7,000 strings. Or the technical writers will suddenly discover a legacy book that simply MUST be translated, two weeks before the planned release date. Be prepared for this to happen, and deal with it as best you can. The best vendors will understand these issues and will cater for them well, but localization vendors have to make their own plans and assign their own resources. The best advice here is: keep your vendor advised of all changes, even tentative ones. Give your vendor the best warning of changes, the moment you know them. Try to avoid exposing yourself to the question: "well, why didn't you tell us?"