Warn on region mismatch between filename locale and target-language

[konradmichalik]

Follow-up to #119.

The XLIFF target-language check currently normalizes both the filename locale and the target-language / trgLang attribute to the base language before comparison. That is intentional and pragmatic, but it silently accepts region-level inconsistencies:

Filename	XLIFF attribute	Current	Desired
de_DE.locallang.xlf	target-language="de-AT"	OK	WARNING
messages.de_DE.xlf	trgLang="de-AT"	OK	WARNING

Proposal

• Keep base-language mismatches as ERROR (current behavior).
• Emit a WARNING when the base language matches but regions differ.
• Lift the locale parse/compare into a small LocaleUtility so other validators can reuse it (suggested by @maikschneider in Validate target-language in XLF files #119).

Out of scope

• Validating that the region itself is meaningful (e.g. de-XX is not a real region). BCP 47 conformance can be a separate ticket if it ever becomes relevant.