You can simply change "Questions" to "All Questions" in this line in lang files.
https://github.com/q2a/question2answer/blob/dev/qa-include/lang/qa-lang-main.php#L115
Once you do that, it'll be changed to All Questions. And to be sure that future update of Q2A do not change it back, you should follow the method for Customizing selected URLs or phrases.
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