Language conversion

- resolves #399
- add conversion rule for language

csaf_2.0/prose/csaf-v2-editor-draft.md

@@ -5679,6 +5679,7 @@ Secondly, the program fulfills the following for all items of:
 
 * type `/$defs/version_t`: If any element doesn't match the semantic versioning, replace the all elements of type `/$defs/version_t` with the corresponding integer version. For that, CVRF CSAF converter sorts the items of `/document/tracking/revision_history` by `number` ascending according to the rules of CVRF. Then, it replaces the value of `number` with the index number in the array (starting with 1). The value of `/document/tracking/version` is replaced by value of `number` of the corresponding revision item. The match must be calculated by the original values used in the CVRF document. If this conversion was applied, for each Revision the original value of `cvrf:Number` MUST be set as `legacy_version` in the convertered document.
 * `/document/acknowledgments[]/organization` and `/vulnerabilities[]/acknowledgments[]/organization`: If more than one `cvrf:Organization` instance is given, the CVRF CSAF converter converts the first one into the `organization`. In addition, the converter outputs a warning that information might be lost during conversion of document or vulnerability acknowledgment.
+* `/document/lang`: If one or more CVRF element containing an `xml:lang` attribute exist and contain the exact same value, the CVRF CSAF converter converts this value into `lang`. If the values of `xml:lang` attributes are not equal, the CVRF CSAF converter outputs a warning that the language could not be determined and possibly a document with multiple languages was produced. In addition, it SHOULD also present all values of `xml:lang` attributes as a set.
 * `/document/publisher/name` and `/document/publisher/namespace`: Sets the value as given in the configuration of the program or the corresponding argument the program was invoked with. If values from both sources are present, the program should prefer the latter one. The program SHALL NOT use hard-coded values.
 * `/product_tree/relationships[]`: If more than one `prod:FullProductName` instance is given, the CVRF CSAF converter converts the first one into the `full_product_name`. In addition, the converter outputs a warning that information might be lost during conversion of product relationships.
 * `/vulnerabilities[]/cwe`: If more than one `vuln:CWE` instance is given, the CVRF CSAF converter converts the first one into `cwe`. In addition, the converter outputs a warning that information might be lost during conversion of the CWE.