Hi, it’s not really a bug but maybe a conception problem.
Here my settings:
- Language: English
- Timezone: Japan
With this settings my Dashboard Gantt chart calendar don’t display the Japanese holiday.
As you can see in the screenshot above it’s in English (rounded parts) and the July 15 is not gray (海の日).
If I change my language to日本語, the July 15 become gray.
[I can only add one picture but you can imagine the 15 in Gray 
]
For me It’s not making sense as the holiday should be related to timezone not to language.
Timezone is used to show where you work, I mean which timezone you work in so Holiday should be set according to. Maybe you have others cases where it make sense to link it to Language. So in this case may need to add a new option to specify which holiday date to apply.
Actually it’s just a suggestion maybe not so much people are affected or complain about and it’s not blocking.
