For the weather conditions description, the translation needs to be in language-es.txt after the 'conditions|begin||' marker like
conditions|begin||
langlookup|Dry|Seco|
langlookup|Light rain|Lluvia débil|
langlookup|Showers|Chubasco|
The plaintext-parser-lang-es.txt is not used here -- it is only used if you are using WXSIM for forecasts.
The Spanish text for the forecasts in WC-forecast.php is provided by TWC/WU via the API. Further translation is not available using the template's language-es.txt .. sorry.