That doesn't look like a katana to me. It doesn't like like any kind of "samurai" blade. It looks like a "Jian", a Chinese sword often used in Tai Chi. The blade is straight rather than curved, and very flexible instead of rigid. The guard is a small rectangle, and there is a tassle a the end of the handle, All of these are characteristics of the Jian, but not anything in the Katana family. In addition, the handle is apparently completely hollow, which means the blade has a very minimal tang, if any at all. Any kind of "samurai style" blade intended for use in battle (WWII) would have had a full tang, or nearly so.
Of course this still doesn't help translate the insert, which is what you asked about. But it looks like a Chinese sword, so that insert is probably also Chinese.