Rename REST API command translate to translations.

This shall fix #1423.

Before renaming, I tested if the command `translations` was free by calling
```
http://localhost:8000/translations/file%3A%2F%2F%2Fprivate%2Ftmp%2FGroups.casl
```
which resulted in 
```
*** Error:
file does not exist: /var/folders/sq/9gmlj7991qq89myy2091r7vh0000gn/T/MyHetsLib/translations/file:///private/tmp/Groups.env
no input found for: translations/file:///private/tmp/Groups.casl
```
So I assume this command was really free.