Fiel vs. leal
The Spanish words 'fiel' and 'leal' both translate to 'faithful' or 'loyal' in English, but are used in slightly different contexts.
Fiel
'Fiel' usually refers to faithfulness or loyalty in relationships, particularly marriages.
Ella es fiel a su marido.
(She is faithful to her husband.)
Ser fiel no es una obligación, es una elección.
(Being faithful is not an obligation, it is a choice.)
Leal
'Leal', on the other hand, often refers more to loyalty in a broad sense, such as loyalty to one’s friends, country or principles.
Los soldados fueron leales a su país hasta el fin.
(The soldiers were loyal to their country until the end.)
Los amigos leales son un tesoro.
(Loyal friends are a treasure.)
Summary
'Fiel' and 'leal' both indicate a sense of loyalty and faithfulness. However, 'fiel' is more used in personal relationships scenarios while 'leal' tends to be used in broader areas such as one's loyalty towards their friends, country, or personal convictions.