I recently enabled Tags feature on the forum, which, I think is a really powerful one and gives more flexibility.
I would recommend using “solved” as tag, not a category. Then it can be applied to any topic. The tag can also be in a different language, which is great too
Can tags, or at least some of them, be colour coded so as to stand out better? I think that would help the “at a glace” determination of a topics status.
In which case would the Solved category be best used for?.. understanding that a “solution” can be rather subjective.
Need to check the coloring. Pretty sure it’s doable.
Ideally, i would get rid of categories as they are kind of old-school “folder” approach, but we already have them, so let them stay.
I resemble that remark 
Seems to be a sensible solution for posts in any language rather than moving them to Solved.
Perhaps we can change Solved to Costas Approved… problem there is all currently Solved ESP-01 topics would need to be eradicated 