RFC: add ENV for locale

add ENV["JULIA_LOCALE"], so we could make use of ENV 
i18n helpdb is put byside of original `helpdb.jl`
```
$ ls usr/share/julia/
base  doc  examples  extras  helpdb.jl  helpdb_zh_CN.jl  test  ui
```

```
julia> ENV["JULIA_LOCALE"]="zh_CN"
"zh_CN"

julia> help(sin)
Loading help data...
Base.sin(x)

   计算 "x" 的正弦值，其中 "x" 的单位为弧度。 # This is Chinese ... 

julia> Base.locale("")

julia> help(sin)
Loading help data...
Base.sin(x)

   Compute sine of "x", where "x" is in radians

julia> Base.locale("zh_CN")

julia> help(sin)
Loading help data...
Base.sin(x)

   计算 "x" 的正弦值，其中 "x" 的单位为弧度。

julia>
```