Griffiths Translation: For, overthrowing what hath ne’er been shaken, thou goest forth alone destroying Vṛtras. For him who followeth thy Law the mountains and heaven and earth stand as if firmly stablished.
Does Rig Veda 3.30.4 say the Earth is immovable?
No, unshakable is the right word instead of immovable.
In the beginning the Earth was shaking or swinging and it's Indra (in some texts) or Vishnu (in some texts) who made Earth unshakable by installing mountains or dividing the Earth in Continents. And today Earth is unshakable, only earthquake occurs sometimes.
Even after the earth was brought up from the seabed and was established above the ocean, it was still swinging like a boat. Narayana then created the mountains to stop the earth from swinging. But the mountains were burnt down by Agni (fire) named Samvartak. The mountains submerged into the ocean. The mountains after having displaced the water had become fixed at their respective places. Therefore, Narayana divided the whole earth into seven islands and created the four lokas just like before. (Source Natural and Flawed Creations chapter of Markandeya Purana)
The below Hindi translation of Rig Veda 3.30.4 seems to be more correct:
HYMN XII. Indra.
HE who, just born, chief God of lofty spirit by power and might became the Gods’ protector,
Before whose breath through greatness of his valour the two worlds trembled, He, O men, is Indra.
He who fixed fast and firm the earth that staggered, and set at rest the agitated mountains,
Who measured out the air's wide middle region and gave the heaven support, He, men, is Indra.
Who slew the Dragon, freed the Seven Rivers, and drove the kine forth from the cave of Vala,
Begat the fire between two stones, the spoiler in warriors’ battle, He, O men, is Indra.
Here's an alternate translation and explanation from the Oxford translation by Stephanie W. Jamison and Joel P. Brereton:
II.12 (203) Indra
15 verses: triṣṭubh
This is one of the better known and most widely anthologized hymns to Indra. Its distinctive rhetorical mark is its refrain, "he, o peoples, is Indra." The "peoples" (jána) refer to the Vedic peoples, and the hymn repeatedly returns to Indra's past victories over non-Vedic beings with the promise of future victories. The hymn portrays Indra as a successful warrior. It begins with his origin (vs. 1) and then refers to his great deeds through which the world was created and made inhabitable (vss. 2–3). The poet then announces Indra's victory over Dāsas, Dasyus, and other enemies and his aid for his worshippers and allies (vss. 4–10). Toward the end of the hymn, the poet mentions Indra's famous victories over his demonized, if not quite demonic, enemies (vss. 11–12). Then he returns to Indra's rule over the cosmos (vs. 13) and his help for those now making ritual offerings to him (vss. 14–15). One verse that has attracted particular attention is verse 5, which says that there are some who wonder where Indra is and who declare that Indra "does not exist." These appear to be Vedic people who question Indra's power and who, in denying Indra and refusing to perform the rituals, approximate the Dāsas, whom Indra defeats (vs. 4). The poet insists that they should be aware of Indra as the "terrifying" (ghorá) one and trust in him. At the end, in 15b, after recounting his great exploits, the poet himself confidently asserts that Indra is indeed real. The reality of Indra may signify not only his existence as a powerful god, but more especially his actual presence at the poet's sacrifice.
Who, even when just born, was the foremost thinker, the god who by his own will tended to the gods, before whose explosiveness the world-halves trembled in fear because of the greatness of his manliness – he, o peoples, is Indra.
Who made firm the wavering earth, who settled the quaking mountains, who gave the midspace wider measure, who propped up the heaven – he, o peoples, is Indra.
Who, having smashed the serpent, let flow the seven rivers, who drove away the cattle by uncovering Vala, who produced the fire between two stones, gathering the winnings in contests – he, o peoples, is Indra.
The same deed is mentioned again in RV II.17:
HYMN XVII. Indra.
LIKE the Aṅgirases, sing this new song forth to him, for, as in ancient days, his mighty powers are shown,
When in the rapture of the Soma he unclosed with strength the solid firm-shut stables of the kine.
Let him be even that God who, for the earliest draught measuring out his power, increased his majesty;
Hero who fortified his body in the wars, and through his greatness set the heaven upon his head.
Thou didst perform thy first great deed of hero might what time thou showedst power, through prayer, before this folk.
Hurled down by thee the car-borne Lord of Tawny Steeds, the congregated swift ones fled in sundry ways.
He made himself by might Lord of all living things, and strong in vital power waxed great above them all.
He, borne on high, o’erspread with light the heaven and earth, and, sewing up the turbid darkness, closed it in.
He with his might made firm the forward-bending hills, the downward rushing of the waters he ordained.
Fast he upheld the earth that nourisheth all life, and stayed the heaven from falling by his wondrous skill.