SCM: remove "test only" warning and enable translation in start dialog

[xLPMG]

Removes the warning that the SCM is for testing only. Also enables the translation for the start dialog, which was commented out for some reason.

Whether this PR can be merged or not is discussed in #4636.

Checklist:

• My code follows the code style of this project.
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (header file)
• I have updated the respective chapter in the Stellarium User Guide
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[github-actions]

Hello @xLPMG!

Please check the fresh version (development snapshot) of Stellarium:
https://github.com/Stellarium/stellarium-data/releases/tag/weekly-snapshot

[github-actions]

Hello @xLPMG!

Please check the latest stable version of Stellarium:
https://github.com/Stellarium/stellarium/releases/latest