So I build Localizely. You upload your localization file for main language, translate content to remaining languages and then download files for all those languages.
When you start developing a new version of your app, you just create a new branch in Localizely. You can compare what keys and translations are changed between branches and optionally merge those.
This is just a start what I want to achieve with Localizely ( https://localizely.com/ ) that is not on the market.
Please let me know if you have any questions.
We can focus more on making our apps better if localization does not create a bottleneck.