The Merriam-Webster Dictionary defines “volunteer” as:
1: a person who voluntarily undertakes or expresses a willingness to undertake a service: as
a : one who enters into military service voluntarily
b (1) : one who renders a service or takes part in a transaction while having no legal concern or interest (2) : one who receives a conveyance or transfer of property without giving valuable consideration
2: a volunteer plant
It gives the origin as “obsolete French voluntaire (now volontaire), from voluntaire, adjective, voluntary, from Old French, from Latin voluntarius. First known use: circa 1600.”
The French word for “volunteer” is bénévole. This comes from the Latin benevolus. Splitting this word into its two parts, it clearly indicates act of doing good to others, therefore kindness or goodwill.
Two English words stemming from the same Latin source, but much less common in English than bénévole is in French, are the noun “benevolence” and the adjective “benevolent.”
The Merriam Webster Dictionary defines “benevolence” as:
1: disposition to do good
2 a : an act of kindness b : a generous gift
3 : a compulsory levy by certain English kings with no other authority than the claim of prerogative
Examples of “benevolence”
1. Self-effacing as well as selfless, he refused all public acknowledgement of his many benevolences to the community
First known use of benevolence: 14th century
The dictionary defines “benevolent” in a similar fashion, e.g. marked by or disposed to doing good”. In practice “benevolent” is used almost exclusively in English in connection with charitable organizations.
In short, the French bénévole is clearly from the same root as the English “benevolent” and “benevolence”, but those English words are not in common use, and the direct translation of bénévole is volunteer.