* remove unnecessary web context data fields, and unify the i18n/translation related functions to `Locale`
* in development, show an error if a translation key is missing
* remove the unnecessary loops `for _, lang := range translation.AllLangs()` for every request, which improves the performance slightly
* use `ctx.Locale.Language()` instead of `ctx.Data["Lang"].(string)`
* add more comments about how the Locale/LangType fields are used
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