In Piwik 2.9.0 (also in 2.8.3), on the Websites Management page, two language strings are missing, producing errors as follows:

(a) (two occurrences on the page)
The string SitesManager_AddSite was not loaded in javascript. Make sure it is added in the Translate.getClientSideTranslationsKey hook.

(b) (displayed in capitals as a column heading)
The string Actions_SubmenuSitesearch was not loaded in javascript. Make sure it is added in the Translate.getClientSideTranslationKeys hook.

Clicking on the message in (a) still produces the desired result (i.e., the link works).

Note: This was a "clean" upgrade/install. Piwik 2.9.0 (and also 2.8.3) was copied into its own server directory, the configuration file and the GeoIPCity files were copied from the previous install, all permissions were set to 755, except the config and tmp directories and contents, which were set to 777 and Piwik was then launched, Superuser logged in, and the upgrade to the existing database was done.