I have discussed this issue internally with SlyCooperFan1 just now.
We don't want to include the Japanese mission subtitles (alternate title) to trivia, because 1) we want to reduce the number of trivia as much as possible for the future, and 2) a lot of missions have Japanese subtitles different to the English mission name, and this includes the latest instalment.
We propose adding a new parameter to the mission infobox dedicated for the Japanese names and the unofficial English translation, separate from the caption parameter. How does that sound to you?
After discussing with User:SlyCooperFan1, we've decided to add a new parameter called Other Names where we will add the mission subtitles in. In the more extreme cases like Ace Combat 6, where there are English and Japanese versions for mission names and subtitles, the Japanese mission name will be added as a footnote, and there will be the official English subtitle and a translated Japanese subtitle in the infobox (Japanese subtitle will be included as a footnote). You can check Sipli Field (mission) page for reference.
The caption parameter will remain as a function to describe the image in the infobox. Any subtitles added in that field will be transferred to Other Names instead.
If you have any problems, please feel free to let us know.