Not sure if the contribute page is the right way. A developer should be able to build and run the code from sources in order to contribute. Indeed you don’t need to use the dev parameters to contribute a patch, but the dev mode is used to ease the debuging of sao code as the build code is not minified.
We don’t care about locales on development. You just only have to take care to mark strings as translatable and they will be uploaded to pootle before the translation start and downloaded from pootle before the version release.