Importing/Exporting Word document from NAV Blob sometimes results in corrupted file

NAV2015 - Build 40459

We have a functionality where users can create word documents for a record. This is stored in a Blob field; nothing special so far.

Here's where it gets interesting:

If I save my document (before importing it) via NAVWordHelper.CallSaveAs:

• If my Word application default language is Dutch (even with English also present) it will work fine;
• If my Word application default language is English (with or without Dutch present), it produces a "Document appears to be corrupted" error

However:
If I save my document (before importing it) via NAVWordHelper.CallSaveAsFormat with wdFormatStrictOpenXMLDocument format:

• If my Word application default language is English (with or without Dutch present) it will work fine;
• If my Word application default language is Dutch (even with English also present), it produces a "Document appears to be corrupted" error

Windows regional settings have no impact on either set of results.

Other information:

• I have not yet tested with other languages, but I would have thought that OpenXmlFormat would be language independent.
• It seems that only Dutch (Netherlands) is causing an issue with the wdFormatStrictOpenXMLDocument - I have tested Arabic (Iraq), Chinese (PRC), Dutch (Belgium) and German and they all seem to work well
• Blob compression settings have no impact.
• The corruption seems to be present in the Styles.xml at least
• The corruption might be linked to collation, since in the corrupted file (Styles.xml) "Colorful" is consistently represented as Col5rful (as in w:name="Col5rful List Accent 4")
• Using TextEncoding other than Windows while importing/exporting the blobs produces I/O errors - most likely because TextMode = FALSE.
• Using TextEncoding in the CreateOutStream/CreateInStream functions likewise has no effect
• Environment is NAV2015 - Build 40459
• I have tried several other methods of storage, but none have proved multilingually sound, so far

Edit: updated for testing of additional languages