By default, the search ignores German umlauts. Searching for “uber” will also find “über” and so on. This ignorance is not a Relevanssi feature but instead governed by your database. The default database collation WordPress uses (
utf8mb4_unicode_ci) ignores all accents, including umlauts.
If you want the search to care about umlauts, you can change the collation. For German, use
Now the database will not ignore umlauts. Relevanssi also removes accents to match what the database is doing, so you must also undo that. Add this to your site:
remove_filter( 'relevanssi_remove_punctuation', 'remove_accents', 9 );
Once you’ve done these steps, rebuild the index. Now the search should not ignore the umlauts.