Soixante-dix vs. septante
The main difference between the French words 'soixante-dix' and 'septante' primarily lies in the location they are used within Francophone regions. Both translate to seventy in English, but are used in different parts of the French-speaking world.
Soixante-dix
'Soixante-dix' which translates to 'sixty-ten', is commonly used in France. It is part of a vigesimal (base 20) counting system particular to French.
J'ai soixante-dix ans.
(I am seventy years old.)
Cette maison a coûté soixante-dix mille euros.
(This house cost seventy thousand euros.)
Septante
'Septante' is mostly used in Switzerland and Belgium as a more directly decimal (base 10) expression for seventy, similar to most other Romanic languages.
Il y a septante personnes dans la salle.
(There are seventy people in the room.)
Elle a atteint l'âge de septante ans.
(She has reached the age of seventy.)
Summary
'Soixante-dix' and 'septante' both denote the number 70 in French, but their usage varies based on regional dialects. 'Soixante-dix', a remnant of the vigesimal system, is mainly used in France while 'septante' follows a more direct decimal system and is mainly used in Switzerland and Belgium.