Fix TimeSpan-related humanization for French culture

[louis-z]

Fixes #973

• Replace "année" with "an"
• Make sure 0 is followed by a singular time unit (not plural, as in English)
• Replace "pas de temps" by "temps nul", which arguably sounds more natural

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[clairernovotny]

Thanks!

[louis-z]

You're welcome. Glad I could help.