Default keyboard on Mac for all apps except one

I'm on a Mac and have three different keyboards setup. I want all my apps to default to English except for Slack where I use Spanish (LA). My problem is that for some reason, Mac defaults all my apps, including browsers to Spanish and I need to change it every time I open a new app. How can I make it so that all my apps have English keyboard by default and only Slack stays on Spanish (LA)?

Question by Jorjani on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.