Add translation support for source data dictionaries

[abhidg]

Currently, only data dictionaries in English are supported. Add support for translation using offline translation packages such as EasyNMT.

[abhidg]

API design, new function translate_dataframe:

def translate_dataframe(df: pd.DataFrame, variable_column: str, source_lang: str | None = None) -> pd.DataFrame:
    "Translate data dictionaries into English"
    # other than variable_column, all fields are translated.
    # categorical fields are translated once to save CPU time and then transformations applied