Allow for IETF region subtag in Language field

[jeroen]

Kurt says on r-devel that any IETF tags are OK, for example pt-BR.

[codecov-io]

Codecov Report

Merging #55 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #55   +/-   ##
=======================================
  Coverage   99.54%   99.54%           
=======================================
  Files          18       18           
  Lines        1100     1100           
=======================================
  Hits         1095     1095           
  Misses          5        5

Impacted Files	Coverage Δ
R/syntax_checks.R	100% <100%> (ø)	⬆️

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[gaborcsardi]

Thanks!