This fixes some issues with language params handling by separating params from configuration values per language.
This means that you can now do this:
```toml
[languages]
[languages.en]
languageName = "English"
weight = 1
title = "My Cool Site"
[languages.en.params]
myParam = "Hi!"
```
This is not a breaking change, but the above is a less suprising way of configuring custom params.
It also fixes some hard-to-debug corner-cases in multilingual sites.
Fixes#4356Fixes#4352
The gain, given the "real sites benchmark" below, is obvious:
```
benchmark old ns/op new ns/op delta
BenchmarkHugo-4 14497594101 13084156335 -9.75%
benchmark old allocs new allocs delta
BenchmarkHugo-4 57404335 48282002 -15.89%
benchmark old bytes new bytes delta
BenchmarkHugo-4 9933505624 9721984424 -2.13%
```
Fixes#2495