Error: Call to undefined function in i18ntaxonomy.module

That function is on i18n.module, so this shouldn't happen. You may have mixed module versions installed somewhere.

Jose, I keep getting the same error after trying to install a new module or uninstall existing modules (or even just clicking on Save configuration). Then nothing works and I have to restore the database from a backup.

Here's the list of modules installed on my website:

Drupal 6.12
AddThis Button 6.x-2.7
Administration menu 6.x-1.4
Advanced help 6.x-1.2
authorship 6.x-dev
Backup and Migrate 6.x-1.2
Better Formats 6.x-1.0
CacheExclude 6.x-2.1
Clean Feeds 6.x-1.x-dev (2008-Oct-20)
Comment Notify 6.x-1.2
Content Construction Kit (CCK) 6.x-2.3
Contrib Toggle 6.x-1.3
Date 6.x-2.2
Read More link 6.x-5.0-rc1
External Links 6.x-1.7
Favicon 6.x-1.0
FCKeditor - WYSIWYG HTML editor 6.x-1.x-dev (2009-May-14)
Footnotes 6.x-2.1
Global Redirect 6.x-1.x-dev (2009-Jun-06)
Google Analytics 6.x-2.2
Table of Contents 6.x-2.x-dev (2009-Mar-18)
Internationalization 6.x-1.x-dev (2009-May-21)
ImageAPI 6.x-1.6
ImageCache 6.x-2.0-beta9
IMCE 6.x-1.2
jQuery Update 6.x-2.x-dev (2009-Apr-23)
LoginToboggan 6.x-1.5
Mime Mail 6.x-1.x-dev (2009-Jun-08)
Mollom 6.x-1.8
Content administration 6.x-1.0-beta2
Nodeauthor information 6.x-1.8
Nodequeue 6.x-2.1
Meta tags 6.x-1.0
Path Redirect 6.x-1.0-beta3
Pathauto 6.x-1.1
Porter-Stemmer 6.x-1.0
Printer, e-mail and PDF versions 6.x-1.7
Scheduler 6.x-1.3
Simplenews 6.x-1.0-rc6
Simplenews Analytics 6.x-1.1
Simplenews Statistics 6.x-1.2
Subscribers Counter 6.x-1.3
Text Resize 6.x-1.5
Token 6.x-1.12
Translation 404 - Translation Page Not Found 6.x-1.2
Translation helpers 6.x-1.x-dev (2008-Oct-09)
Translation Overview 6.x-2.2
Translation status 6.x-1.0
Vertical Tabs 6.x-1.0-beta3
Views 6.x-2.5
Webform 6.x-2.7
XML Sitemap 6.x-1.0-beta5

I'm wondering if any other module than i18n may affect it... Also, update.php has been run after making recent updates. I used both i18n 6.x-1.0 and 6.x-1.x-dev versions. Any thoughts?

Everything works fine now. It was the Contrib Toggle 6.x-1.3 module malfunctioning and turning off some of the Drupal core modules (like Taxonomy) when pressing the Save Configuration button on module page. As the Taxonomy module got disabled, the I18n Taxonomy was throwing an error. After disabling Contrib Toggle everything went back to normal.

Ok, thanks for reporting.