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If we can't change the tags before launch, but change them immediately after, how long does it take Google to recognize the change and adjust our ranking? Will we be digging ourselves out of a hole if we implement it the wrong way and fix it shortly after?

No, it doesn't change my answer, but it's a good distinction to make. It sounds like the international expansion is in process. If the client needs geo-targeted content (sounds like they might), the countries need to be treated differently. Each subfolder as it's own site really. But it sounds like the translation is a good place to start for the time being. For the mobile, my answer remains the same. There isn't anything other than making the translated content mobile or responsive that will help the traffic.