In the strictest possible sense co-citizen would be the English variant of concitadin, however the popular usage of citizen (a person who lives in a particular town or city, originally a concatenation of the word city and denizen,
citizen) actually makes citizen mean the same as national (someone who officially belongs to a particular country). Therefore in correct English I would be a citizen of Glasgow if I lived in Glasgow and a British national if I lived in Britain.
However for common English you would use countryman, countrywoman or compatriot for conacional (depending on the level of specificity and tone you wish to convey), and fellow town variant descriptor: for example fellow Londoner, fellow Glaswegian, fellow Aberdonian etc. to describe someone as being from the same city/town as yourself.
Using the word
fellow can add to the meaning and indicate that you share the status you have attributed to them, and are stating the that you and the people mentioned are members of a group (abstract or real):
fellow citizens (the person stating this is also a citizen in the same locale) vs.
citizens (the person stating this is mentioning a group who are citizens, but not stating whether they are or not).
Hope that helps:)