Fix data race and optimize performance

[nicksnyder]

fixes #181
helps #182

[codecov]

Codecov Report

Merging #183 into master will increase coverage by 0.57%.
The diff coverage is 93.93%.

@@            Coverage Diff            @@
##           master    #183      +/-   ##
=========================================
+ Coverage   82.33%   82.9%   +0.57%     
=========================================
  Files          14      14              
  Lines        1285    1293       +8     
=========================================
+ Hits         1058    1072      +14     
+ Misses        161     158       -3     
+ Partials       66      63       -3

Impacted Files	Coverage Δ
i18n/message_template.go	92.85% <100%> (+17.85%)	⬆️
internal/template.go	90.47% <90.47%> (+12.69%)	⬆️

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