These "defaultcodec" values in the translation files are not used by Qt, which issues a warning about it. I see that the "defaultcodec" value is not used in Bitcoin these days, so it is clearly not necessary. Also, the translation defaults to LATIN-1, but MintCoin must be using a proper UTF-8 codec, otherwise the Russian translation would not work.
These "defaultcodec" values in the translation files are not used by Qt, which issues a warning about it. I see that the "defaultcodec" value is not used in Bitcoin these days, so it is clearly not necessary. Also, the translation defaults to LATIN-1, but MintCoin must be using a proper UTF-8 codec, otherwise the Russian translation would not work.