set the LANG env when rendering rmarkdown

[shrektan]

What problem did you solve?

Closes #933

The process.env doesn't contain the LANG environment variable. Thus, when the child process calls R, R will set the locale to "C". This causes an issue that UTF-8 strings can't be corrected handled.

(If you have)Screenshot

(If you do not have screenshot) How can I check this pull request?

Open the below Rmd file in VSCode. Click the knit button to render the file. View the generated HTML file you will see:

(I only verified this on macOS and Windows. On Windows, setting the LANG won't affect the locale anyway.)

1. Before: The locale is "C" and there's no string or garbaged string in the left-bottom corner.
2. After: The locale is "xxx_UTF-8" and the left-bottom displays "科比", as expected.

---
title: "minimal example"
author: "Kobe Bryant"
output: html_document
---

```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = F, warning = F, message = F, eval = T)
```

Chinese character in the main article can be rendered normally.
(正文区的中文可以正确的渲染)

Figure legends cannot be rendered normally.

```{r, fig.cap = "科比"}
print(Sys.getlocale())
plot(1:5, 1:5)
```

[renkun-ken]

Works as expected. LGTM. Thanks!