Wrong language in text obtained via SSI.php

[_Jojo_]

I'm using a SSI script that runs via cli (so not triggered by a potentially logged-in user's session) to create a list of recently updated topics to show on my website. It calls ssi_recentTopics and nothing else.
The forum has English and French locales installed, and recently I have observed that sometimes the topic list generated by SSI.php uses French localization for the date components (there's only very few users who set the forum locale to French).
I suppose that there is some internal caching happening and that this happens when the last user that caused the cache to be updated happened to use the French forum locale.
Is this a bug? If not, how can I force SSI.php to always use the English locale?
I'm running SMF 2.1.4.