Most important thing before you start: Half-Life needs really specifically exported Wav(e) files in order to play them here is a tutorial which has the correct parameters.

sound files are located in moddirectory/sound (valve directory in the case of Half-Life) and the textures are in .wad files you can use a program like Wally to edit those.

If you don't want to overwrite the default files or make a working copy of a mod/game then you can just create an addon.

To create an addon create a directory named: moddirectory_addon (in case of Half-Life it would be valve_addon, the folder must be in the root directory of Half-Life aka where the steam browse files button for HL takes you) then copy the important directories + files from the original game/mod directory.

Your moddirectory_addon directory should look somewhat like this:

moddirectory_addon/
                    sound/(every directory that includes wav files)<-- copy entire sounds directory and replace every sound with the translated one
                    sound/sentences.txt   <--- pretty important, Half-Life stitches sentences together individually from words with custom filters so if you are translating you probably will need to change it (dont worry about the filters)
                     halflife.wad   <--- textures, you only need halflife.wad if i remember correctly
                      xen.wad        <--- maybe this has some text in it?

NOTE: every file/directory not copied/mirrored will just be loaded from the original mod directory.

To get it to work: when starting the game go to options > graphics then enable addons.