the "Keywords" feature is intended for adding keywords to html pages, i.e. FirstSpirit content pages. The CMS_INPUT_TEXT element is supposed to be added to page templates and it is vital to use the "useLanguages" parameter for supporting multi-lingual values.
In your case, you have added the input component to the Metadata template. Metadata, however, is not language-dependent (see FirstSpirit Online Documentation - Using variables in metadata ). So I do not think that adding keywords to media can be achieved with this feature. I am not sure what happens when you try to add the "useLanguages" parameter to CMS_IMPUT_TEXT - maybe you want to try it out and see whether the locale attribute gets added to the "Keywords" element during the generation of the portal_search.xml. Without the locale, the xml will not be processed and activation fails as you have already noticed.
It would be interesting to know how you plan to access the keywords in the SAP Portal. The "Keywords" feature was designed to work together with TREX and standard KM capabilities (searchable properties in a KM namespace). But as I can see, you are already creating "keywords" inside a "generic attribute" element. If the search component is correctly configured, a custom property called "keywords" should be added to the KM resource. In case you have a custom search solution, you may succeed in reading the keywords from there and do not need the "Keywords" element at all.