Sends PropertyIsEqualTo for most fields;
gmu_code_desc uses PropertyIsLike (wildcards: *abc*).
Values are in the active language.
How it works
The FILTER= WMS vendor parameter carries OGC
FilterEncoding XML. MapServer evaluates it server-side against
the PostGIS view — only matching polygons are rendered per tile.
The lang=fr|de runtime substitution in the mapfile
switches the attribute language of tecto_desc,
chrono_desc, gmu_code_desc, and litho_desc.
Filter values must match the active language.