Starting from a dataframe df:

  U   L
111  en
112  en
112  es
113  es
113  ja
113  zh
114  es

Imagine you want to add a new column called S taking values from the following dictionary:

d = {112: 'en', 113: 'es', 114: 'es', 111: 'en'}

You can use map to perform a lookup on keys returning the corresponding values as a new column:

df['S'] = df['U'].map(d)

that returns:

  U   L   S
111  en  en
112  en  en
112  es  en
113  es  es
113  ja  es
113  zh  es
114  es  es