-Copy the English localization file.
-Replace the English translation by your own.
-When your language has different prefixes for each of the chesspieces,
you can solve this with a build in system to handle this. For instance if you look at the Spanish
localization (es_ES.lang), there is a line:

entity.MineChess.Rook.replacement.#=la

This line means that in every sentence where there is a Rook mentioned, everywhere where a '#' appears,
this will be replaced by 'la'. You can use the following characters: '@', '#', '$', '^', '&' and '*'.