Improved schematron localisation

[joachimnielandt]

In the dcat-ap schema plugin there is a need to translate schematron strings. One of the ways is to use the loc/eng/schematron-rules-x.xml files that are loaded by iso_svrl_for_xslt2.xsl. A helper function replacePlaceholders was added that makes sure the schematron rules remain language agnostic and the translations are easily adaptable to the needs of each language.

• English is proposed as the fallback language whenever a file cannot be found.
• A shared schematron localisation file is introduced to prevent having to duplicate common schematron translation strings. Not having this file in place works fine.
• Fix in iso_svrl_for_xslt2.xsl to make sure $loc is also used when wrapped in a function.

In the schema plugin a PR was created that makes use of this change: metadata101/dcat-ap#38.

@fxprunayre @CMath04

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[CMath04]

Tested and it works fine.
I also check for regression on ISO but it all seems fine.