Adapt code for L10nRegistry API changes in 79
There was a L10nRegistry.registerSource
-> L10nRegistry.registerSources
change in https://bugzilla.mozilla.org/show_bug.cgi?id=1648631.
There was a L10nRegistry.registerSource
-> L10nRegistry.registerSources
change in https://bugzilla.mozilla.org/show_bug.cgi?id=1648631.