https://community.sap.com/t5/crm-and-cx-q-a/polish-language-to-solr/qaq-p/12021384

HunspellStemFilterFactory which the current version supports

https://support.sitecore.com/kb?id=kb_article_view&sysparm_article=KB1001661

To add a new language to solr, you need put additional fields in schema.xml file under yoursolrserver/solr/server/multicore/conf folder. This is achieved by copy-pasting what's already there for existing languages, for example the text fieldtype :

 <fieldType name="text_pl" class="solr.TextField" positionIncrementGap="100">
             <analyzer>
                 <tokenizer class="solr.StandardTokenizerFactory" />
                 <filter class="solr.StandardFilterFactory" />
                 <filter class="solr.LowerCaseFilterFactory" />
                 <filter  class="de.hybris.platform.solrfacetsearch.ysolr.synonyms.HybrisSynonymFilterFactory" ignoreCase="true" synonyms="pl" coreName="${solr.core.name}"/>
                 <filter class="solr.WordDelimiterFilterFactory"
                     generateWordParts="1" generateNumberParts="1" catenateWords="1"
                     catenateNumbers="1" catenateAll="0" splitOnCaseChange="0" />
                 <filter  class="de.hybris.platform.solrfacetsearch.ysolr.stopwords.HybrisStopWordsFilterFactory" ignoreCase="true" lang="pl" coreName="${solr.core.name}"/>
                 <filter class="solr.StopFilterFactory" words="stopwords_pl.txt" ignoreCase="true" />
                 <filter class="solr.ASCIIFoldingFilterFactory" />
                 <filter class="solr.SnowballPorterFilterFactory" language="Polish" />
             </analyzer>
         </fieldType>

Implementacja

/opt/tomcat/webapps/opencms/WEB-INF/solr/configsets/default/conf

/opt/tomcat/webapps/opencms/WEB-INF/solr/configsets/default/conf/schema.xml

    <!-- Polish -->
    <fieldType name="text_pl" class="solr.TextField" positionIncrementGap="100">
      <analyzer>
        <tokenizer class="solr.StandardTokenizerFactory"/>
        <filter class="solr.LowerCaseFilterFactory"/>
        <!-- <filter class="solr.StempelPolishStemFilterFactory"/> -->
      </analyzer>
    </fieldType>

    <!-- Russian -->
    <fieldType name="text_ru" class="solr.TextField" positionIncrementGap="100">
      <analyzer>
        <tokenizer class="solr.StandardTokenizerFactory"/>
        <filter class="solr.LowerCaseFilterFactory"/>
        <!-- <filter class="solr.StempelRussianStemFilterFactory"/> -->
      </analyzer>
    </fieldType>

    <field name="text_pl"             type="text_pl"      indexed="true"  stored="false" multiValued="true"/><!-- Catchall for PL text fields -->
    <field name="text_ru"             type="text_ru"      indexed="true"  stored="false" multiValued="true"/><!-- Catchall for RU text fields -->

    <dynamicField name="*_pl"         type="text_pl"      indexed="true"  stored="true" />
    <dynamicField name="*_ru"         type="text_ru"      indexed="true"  stored="true" />

    <copyField source="*_pl"      dest="text_pl"/>
    <copyField source="*_ru"      dest="text_ru"/>