fix l10n script

[goatshriek]

Fixes the functionality of the add_l10n_string.rb script and localizes the error message for severity colors with non-stream targets, as this was not added due to the broken script. This was identified during work on #427.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.85%. Comparing base (19d83b4) to head (46fbdca).

Additional details and impacted files

@@           Coverage Diff           @@
##           latest     #430   +/-   ##
=======================================
  Coverage   90.85%   90.85%           
=======================================
  Files          47       47           
  Lines        4384     4384           
  Branches      586      586           
=======================================
  Hits         3983     3983           
  Misses        269      269           
  Partials      132      132           

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