add instructions to docs about testing translations

I just learned how to do this, so it's helpful to tell other's how!

docs/dev/front/test_translations_in_devapp.md

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+# How to Specify Translation Source
+
+When adding a language or testing translation updates, it can be useful to see changes in the devapp. Follow the steps bellow to point your devapp to a specific branch for translations. 
+
+In the GitHub CLI for your e-mission-phone repo, navigate into the locales folder:
+
+`cd locales`
+
+Clear the remote tracking, and set the appropriate upstream and origin repositories:
+
+`git remote rm origin`
+
+`git remote add upstream https://github.com/e-mission/e-mission-translate`
+
+`git remote add origin https://github.com/[your_account]/e-mission-translate`
+
+Check what has been set:
+
+`git remote -v`
+
+Checkout the branch you wish to test
+
+`git checkout --track origin/[branch_to_test]`
+
+Now, the translations you see in the emulator will be pulled from that branch, and your edits in locales can be pushed into your fork/branch of translations.