Improve template zshrc

This improves `templates/.zshrc` for two reasons:
* mention that users may need to set `$LANG` (see #1286 and #1823)
* mention that users can use another folder than `$ZSH/custom` (thanks to #525) by setting `$ZSH_CUSTOM` (see #1628, #1989, #2295)

Additionally, I think `$ZSH` should be referred to as "oh-my-zsh *installation* folder" instead of *configuration*.