Here is my values\\strings.xml (the default file) and everything is self explanatory:
My question(s):
Just make sure that you have same naming conventions on all strings files including the same capitalization. let's say if you have "sign_in" in strings.xml and "sign_In" in your fr/string.xml, so it will give you error on fr/strings.xml like "is translated here but not found in default locale". So, when you edit to "sign_in". The error will remove.