Add Language Filter to Projects Page

**Is your feature request related to a problem? Please describe.**
Users often want to contribute to projects that use languages they're comfortable with, but it's hard to quickly find projects of preferred language on the main projects page.

**Describe the solution you'd like**
A filter on the projects page that lets users select one or more programming languages and instantly see only the projects that match. This would be a dropdown with checkbox list, so it's easy to use and works well with other filters.
My planned approach is to add language as a facetable attribute and extend the search hook on the frontend to support filtering via language on the projects page.

**Are you going to work on implementing this?**
- [x] Yes
- [ ] No

**Additional context**

https://github.com/user-attachments/assets/a3106312-ee18-464e-b924-4d154c5ba9ac

(uses AND logic if multiple languages are selected)

Start at the projects page and trace the frontend search hook, then inspect how existing facetable attributes and filters are wired. Implement the language checkbox filter with AND behavior for multiple selections, and verify that it combines correctly with the other project filters.

Written by the indexing model from the issue text.