Download the XXX.mpq of the language you'd like to switch to and copy it into the following directory:

<Diablo 2 installation directory>\D2SE\CORES\<Version>\Language_XXX\XXX.mpq (overwrite the existing file)
OR
<Diablo 2 installation directory>\MODS\<Your Mod>\Language_XXX\XXX.mpq

In D2SE select <Version> (or <Your Mod>) and choose the language in the "Select Option:" menu and click "Start Plugin".


PS: Copying the mpq into a mod directory will not translate the mod itself but only the underlying Diablo II. 
    If the Mod itself doesn't offer the chosen translation there will be a language mix, untranslated strings and maybe even crashes or other problems.

PPS: 1.14 tbl files were used so this should work with all available patches but I've not tested it.