contribute/translate.rst: I am Mikaela! (And other fixes).

* Fix typos
    * `I already write` --> `I already wrote`.
    * `Simple isn't` --> `Simple isn'it it?`
    * `Go back on` --> `Go back to`
    * `is)files` --> `is) files`
    * remove swearing.
* Clone the `testing` branch in the beginning, don't require checkout.
    * Added notice on not having to git checkout if testing was cloned.
* Rename `progval` to `upstream`
    * If people are contributing to multiple projects, the remote
    `upstream` is easier to remember than random nickname.
* I am Mikaela
    * and I use git by myself nowadays.