Translation on month in caption of calendar widget fails

As posted before in https://github.com/polylang/polylang/issues/464

Steps to reproduce the issue

1. Activate Calendar Widget
2. caption of wp-calendar (month) doesn’t get translated
3. Disable PolyLang and the caption is translated

Expected behavior and actual behavior

When I follow those steps, I see the caption (month) of the calendar widget translated with PolyLang activated.

Something with fetching the translation of the month broken but I cannot get my head around, what it is.

Any advice – except of “deactivate PolyLang” welcome!

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