We are migrating the existing japanese website and relaunching in our company platform. I need to do the content testing in the migrated site. Please help me how to verify the Japanese content with the legacy site.
For a one time move it may make the most sense to do this manually.
Put together a list of current functionality and content and verifies that it exists in the new system.
You'll need someone who knows Japanese. That's not really avoidable until you are willing to take on considerable risk. Websites that are translated by people who don't understand the language have a high risk of being wrong.
If you don't have / don't have access to, a translator either get one (think the internet) or state that the business needs to provide one or provide for the cost of one. Throwing some figures out: if the cost is $1000, what is the expected Revenue from the japanese site over the coming year? $300? Don't bother. $300,000? you better bother.