Apple - "say" in different language?

You can use the following command in the Terminal to get a list of all the available voices:

say -v ?

On my system the output looks like this with the voice name followed by the language abbreviation (I removed the comments):

Agnes en_US

Albert en_US

Alex en_US

Alice it_IT

Alva sv_SE

Amelie fr_CA

Anna de_DE

Bad News

Bahh en_US

Bells en_US

Boing en_US

Bruce en_US

Bubbles en_US

Carmit he_IL

Cellos en_US

Damayanti id_ID

Daniel en_GB

Deranged en_US

Diego es_AR

Ellen nl_BE

Fiona en-scotland

Fred en_US

Good News

Hysterical en_US

Ioana ro_RO

Joana pt_PT

Junior en_US

Kanya th_TH

Karen en_AU

Kathy en_US

Kyoko ja_JP

Laura sk_SK

Lekha hi_IN

Luciana pt_BR

Maged ar_SA

Mariska hu_HU

Mei-Jia zh_TW

Melina el_GR

Milena ru_RU

Moira en_IE

Monica es_ES

Nora nb_NO

Paulina es_MX

Pipe Organ

Princess en_US

Ralph en_US

Samantha en_US

Sara da_DK

Satu fi_FI

Sin-ji zh_HK

Tessa en_ZA

Thomas fr_FR

Ting-Ting zh_CN

Trinoids en_US

Veena en_IN

Vicki en_US

Victoria en_US

Whisper en_US

Xander nl_NL

Yelda tr_TR

Yuna ko_KR

Zarvox en_US

Zosia pl_PL

Zuzana cs_CZ

This is an example on how to use say with a german voice, as suggested by scottishwildcat in the comments:

say -v Anna 'Guten Tag!'

In case you want to hear the example list, you can copy this for loop into your terminal, which will result in all installed voices speaking a text example:

say -v \? | while read LINE
 do
  SPEAKER=$(echo $LINE | egrep -o "^[a-zA-Z\-]*[ ]?[A-Z][a-z]+")
  TEXT=$(echo $LINE | egrep -o "#.*" | tr "#" " ")
  echo $SPEAKER
  echo -ne "This is voice $SPEAKER speaking the example text: \n" $TEXT | say -v $SPEAKER
 done

For Mac OS X 10.7, Swedish works pretty well by just downloading the Swedish voices using Preferences->System Voice->Customize and setting either as System Voice. It neatly nails even pretty peculiar Swedish words and spellings.

No guarantee for other languages.

As far as I know the only way to get a Mac to speak in another language is to get voice resources in your target language. A web site for French blind people recommends iVox.

Sorry, no Slovak there I can see.