source: Cache language config

source: Cache language config

```
benchmark                                                                                                                     old ns/op     new ns/op     delta
BenchmarkSiteBuilding/frontmatter=YAML|num_root_sections=1|num_pages=5000|tags_per_page=0|shortcodes=false|render=false-4     552742744     501838152     -9.21%

benchmark                                                                                                                     old allocs     new allocs     delta
BenchmarkSiteBuilding/frontmatter=YAML|num_root_sections=1|num_pages=5000|tags_per_page=0|shortcodes=false|render=false-4     3047393        2957378        -2.95%

benchmark                                                                                                                     old bytes     new bytes     delta
BenchmarkSiteBuilding/frontmatter=YAML|num_root_sections=1|num_pages=5000|tags_per_page=0|shortcodes=false|render=false-4     356533864     354291924     -0.63%
```