Lashon HaKodesh makes Bnei Yisrael Hashem's witnesses

על מעשה דור הפלגה נאמר זכור ימות עולם כו' שנתרחקו כל האומות מלשון הקודש ונשאר רק לבני ישראל.

Concerning the episode of the Generation of the Dispersion it is said, "Remember the days of old" (Devarim 32:7), that all the nations became distanced from Lashon HaKodesh, and it remained only for Bnei Yisrael.

The Sefas Emes explains that at the dispersion the nations lost the holy tongue, which was preserved exclusively for Bnei Yisrael.

כי לשון הקודש שורש כל הלשונות.

For Lashon HaKodesh is the root of all languages.

The holy tongue is the source from which all seventy languages branch out.

כי עיקר יתרון המדבר בכח הנשמה דכ' ויפח כו' לנפש חי' לרוח ממללא והוא לשון הקודש ומזה יש התפשטות כל ע' לשונות כמ"ש יתן אומר המבשרות צבא רב ונשאר לבנ"י שיש בהם הנשמה.

For the essential distinction of the "speaker" lies in the power of the neshamah, as it is written "and He breathed... into a living soul" (Bereishis 2:7), which Onkelos renders "a speaking spirit," and that is Lashon HaKodesh; and from it spread all seventy languages, as it says, "Hashem gives the word; those who bear the tidings are a great host" (Tehillim 68:12) — and it remained with Bnei Yisrael, who possess the neshamah.

Man's superiority is the power of speech rooted in the divine neshamah ("a speaking spirit"), and that root-speech is Lashon HaKodesh. All other tongues derive from it, but it stays with Bnei Yisrael, who carry the full neshamah.

וזה עצמו אות לבנ"י שהם חלק ה' חבל נחלתו.

And this itself is a sign for Bnei Yisrael, that they are the portion of Hashem, the lot of His inheritance.

Possessing the holy tongue is the very proof that Bnei Yisrael are Hashem's own portion, "chelek Hashem amo."

ולכן הם מיוחדים להעיד על הבורא כי כל כח המדבר שניתן לאדם כדי להעיד על השי"ת.

Therefore they are uniquely fit to testify about the Creator, for the entire power of speech that was given to man was given in order to bear witness about Hashem Yisborach.

The purpose of human speech is to testify to Hashem, so the nation that retained the holy tongue is the one fit to be His witnesses.

וז"ש אתם עדי נאום ה' שיש לכם זה הלשון הקודש ונקרא נאום ה'.

And this is the meaning of "You are My witnesses, says (ne'um) Hashem" (Yeshayahu 43:10) — that you possess this Lashon HaKodesh, which is called "ne'um Hashem (the utterance of Hashem)."

Bnei Yisrael can be Hashem's witnesses precisely because their speech is the holy tongue, itself termed "the utterance of Hashem."

וכ' בעד על פי שני עדים.

And it is written regarding testimony, "By the mouth of two witnesses" (Devarim 19:15).

The Sefas Emes now connects to the halachos of valid testimony.

דרשו חז"ל שלא ישמעו סנהדרין מפי המתורגמן.

Chazal expounded that the Sanhedrin must not hear testimony through an interpreter (Sanhedrin 17a).

Valid edus must reach the court directly, not via a translator.

וכל הלשונות הם בחי' תרגום.

And all the [other] languages are in the category of "translation."

Every language other than Lashon HaKodesh is essentially a "targum," a derived rendering rather than the original.

אבל העדות צריך להיות בלשון הקודש:

But the testimony must be in Lashon HaKodesh.

True testimony about Hashem must be borne in the original holy tongue — which is why Bnei Yisrael, who alone retain it, are His witnesses.

Summary: At the Dispersion the nations lost Lashon HaKodesh, the root of all languages and the original "speaking spirit" breathed into man. It remained with Bnei Yisrael, who possess the full neshamah. Since the purpose of speech is to testify to Hashem, and valid testimony cannot be given through an "interpreter" (all other tongues being mere "translation"), Bnei Yisrael alone, bearing the holy tongue, are Hashem's true witnesses.