add checks

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We could add some checks, to mind come:

• a schema for world and area files
• are we using undefined translation keys? should be error
• are there translation keys we are not using (yet?) could be warning
• referenced are not loaded. An area that is referenced in an active/loaded area has to be present in world.toml. Areas might be referenced in e.g. gateways.
• spelling/typography. We could check for spelling mistakse (pun intended) or typographical mistakes in translation files (e.g. using ... is wrong, it is …)

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#17