intl-zanata-*.sh script allow inheriting ZANATA_LOCALES

Only set the `ZANATA_LOCALES` envvar in the scripts if it is
not already defined.  This allows us to interact with zanata
using a specific set of locales that may be different from default.

For example, to pull just the Spanish translations:

  > yarn intl:extract
  > ZANATA_LOCALES=es yarn intl:pull
  > yarn intl:apply

Author: sjd78

scripts/intl/intl-zanata-pull.sh

@@ -1,7 +1,7 @@
 #! /usr/bin/env bash
 [[ -e package.json ]] || cd ../..
 
-ZANATA_LOCALES=de,es,fr,it,ja,ko,pt-BR,zh-CN,cs,ka
+ZANATA_LOCALES=${ZANATA_LOCALES:-de,es,fr,it,ja,ko,pt-BR,zh-CN,cs,ka}
 
 mvn \
     at.porscheinformatik.zanata:zanata-maven-plugin:4.7.8:pull \

scripts/intl/intl-zanata-push.sh

@@ -1,7 +1,7 @@
 #! /usr/bin/env bash
 [[ -e package.json ]] || cd ../..
 
-ZANATA_LOCALES=de,es,fr,it,ja,ko,pt-BR,zh-CN,cs
+ZANATA_LOCALES=${ZANATA_LOCALES:-de,es,fr,it,ja,ko,pt-BR,zh-CN,cs}
 
 mvn \
     at.porscheinformatik.zanata:zanata-maven-plugin:4.7.8:push \