Always fallback to en_US for spellchecking when no lang is available on Linux

[alexwykoff]

• Did you search for similar issues before submitting this one?
  Yes

• Describe the issue you encountered:
  Spellcheck underline did not display in 0.13.1

• Platform (Win7, 8, 10? macOS? Linux distro?):
  Tested all platforms, regression on Linux

• Brave Version (revision SHA):
  Discovered in 0.13.1, regression started in 0.13.0

• Steps to reproduce:

  1. visit www.wordcounter.net
  2. misspell some words in the input field
  3. check for underlines

• Actual result:
  Currently no underlines or context menu suggestions appear

• Expected result:
  Misspelled words should be underlined, right-clicking on the word should give suggested alternatives.

• Will the steps above reproduce in a fresh profile? If not what other info can be added?
  Yes

• Is this an issue in the currently released version?
  Yes

• Can this issue be consistently reproduced?
  Yes

• Extra QA steps:
  1.
  2.
  3.

• Screenshot if needed:

• Any related issues:

[bbondy]

This is actually working the same for me in 0.12.15 and 0.13.0 if you have default lang en_US configured, but for me I had en_CA and so it wasn't working in both 0.12.15 and 0.13.0. I did a fix to always fallback to en_US. I would have thought you'd of been on en_US though. So let's see if it fixes it for you too, but it definitely works and fixes it for me.

[bbondy]

This never worked even on 0.12.15 and before by the way.
The original issue reported here is being fixed in
#7015