back-translations question could trigger and suggest that you use similar procedures for all translations. We assume you have two concerns: (1) making sure you obtain all rights from translators and (2) protecting your translations from infringement. Here's the 411.
Obtaining all rights from translators. You mention that you use work for hire or copyright assignments. We'll guess you're using assignments with contractors, and work-for-hire arrangements for employees. Just so you know, there is a difference. With an assignment, the translator could reclaim rights after 35 years. That's not possible with a work made for hire. Also, the commissioned work-for-hire rules specifically include translations as one of the enumerated categories for independent contractors. Finally, a work-for-hire arrangement allows the hiring party (your company) to be named as author, not the individual translator. Your company should take advantage of these rules and use work for hire agreements when commissioning translations. (You can always include an either/or provision, as we discussed in a previous post.)
Protecting your translations. Translating a copyrighted work requires permission. The resulting translation is entitled to a separate copyright as a derivative work and the translator is the author of the work. Translators routinely assign these rights back to the content owner. (There is more information here and here.) As you probably know, the owner of a translation copyright who wants to register it, has to disclaim the source material.