Customized strings in old studio version applies to active language

[zepumph]

This issue is to make sure that whatever work in done in phetsims/joist#899 that should be in ph-scale, gets into it. Marking as blocking. No work to be done until I note it.

[samreid]

@zepumph and I cherry-picked this and confirmed it works well. We recommend @Nancy-Salpepi double check it in RC.2 following the directions in phetsims/joist#899

[zepumph]

I confirmed in ph-scale and basics 1.6.0-rc.2 that this is working. Ready for close after QA verification

[Nancy-Salpepi]

Following the steps in phetsims/joist#899, the sim opens in the current language that is set in studio 1.6 instead of the language it was saved in when using version 1.5.