If on thesaurus node is configured for more than one locale, the thesaurus gets
activated for all defined locales but can only be used in the first one (all
other locales report a "word not found" error, if thesaurus is accessed for any
word).

See issue URL for a test case. 
Install the extension, enter some German word (e.g. "Haus"), call Thesaurus and
use language German(Germany) -> do word will be found.
User language German (Austria) for the same Word -> suggestions are displayed.

The problem affects only the thesaurus, spell checking and hyphenation are ok.

the node is defined with:

        <node oor:name="ThesDic_de-DE" oor:op="fuse">
            <prop oor:name="Locations" oor:type="oor:string-list">
                <value>%origin%/thes_de_DE_v2/th_de_DE_v2.dat</value>
            </prop>
            <prop oor:name="Format" oor:type="xs:string">
                <value>DICT_THES</value>
            </prop>
            <prop oor:name="Locales" oor:type="oor:string-list">
                <value>de-AT de-DE</value>
            </prop>
        </node>


as workaround you can define two nodes using the same thesaurus location - one
node per locale.