We are trying to not create language-based filters to avoid becoming another Twitter, where you only see people of your language + English. The goal is to be global. We need to talk to each other. And hopefully automate translations help us get there.

could we get manual selection of source lang cos many times it's not detected ( i.e. non-English not detected, or detected but incorrectly (CHS/JPN etc)?

I understand the intention but it doesn't seem practical from my limited experience. New users respond better to people who speak their language, irrespective of auto-translate. Communities based on languages are inevitable and it's good that atleast on Amethyst non-English hashtags are supported that can help with that.