I have heard about the longest location name in Europe which is Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch in Wales and I have found out that it actually means "St Mary's church in the hollow of the white hazel" (according to Wikivoyage). It made me wonder if Welsh can be categorized as an agglutinative language?
No. First, that is a made-up name; but this is not relevant since the compounding it uses is normal in Welsh, just not to this ridiculous degree.
Secondly, this is a compound, like well-known German examples such as die Windschutzscheibewischenersatzgummistreife.
"Agglutinative" does not usually refer to compounding (multiple roots) but the use of multiple bound morphemes to modify a single (or compound) root, such as the Turkish example odanızdakilerimden which contains only the root oda ("room") plus the bound morphemes -nız ("your"), -da ("in"), -ki (nominaliser "thing which"), -ler (plural), -im ("my"), -den ("about"): "about those things of mine in your room".