Is Sanatana Dharma anti-foreigners?
No, most certainly not.
For example, the following mantra from Shukla Yajur Veda clearly states that the holy mantras should be spoken among all men including strangers. Here, the word stranger denotes people who are alien to our Vedic culture. So, the Vedic knowledge should be spread among them too. No question of discrimination or hostility towards those who are unfamiliar with our culture.
YathA imAm vAcham kalyAnim AvadAni janebhyah (1) BrahmarAjanyAbhyAm
sudrAya cha AryAya cha (2) SwAya cha aranAya cha (3)
May i speak the sacred word to the masses of the people (janebhya) (1)
to the brahmana, kshatriya, to the sudra and the Arya (2) and to our
own men and the strangers (3).
Shukla Yajur Veda 26.2
Similarly, the following mantra from Atharva Veda acknowledges the fact that on earth there will be people who will be having a variety of religions and who will speak a variety of languages. The mantra then asks from Mother Earth to be benign to and resourceful for every such people and community irrespective of where they live.
Janam vibhrati vahudA vichasam (1) NAnAdharmAn prithvi (2) YathA
okasham (3) Sahasram dhArA dravinasya me duhAm (4) Dhruveva dhenuh
Earth has people who speak various tongues (1) and those who have
various religions (2), according to their places of abode (3), [May
she] pour for me treasures in a thousand streams (4), like a constant
cow that never fails (5).
Atharva Veda 12.1.45.
Concord among nations is also something being spoken of in the Vedas. One such mantra is the following.
SamjyAnam nah svebhih (1) SamjyAnamaranebhih (2) SamjyAnamashivanA
yuvamihAsmAsu ni yacchatam (3)
Let us have concord with our own people (1), and concord with people
who are strangers to us (2); Asvins, create between us and the
strangers a unity of hearts (3).
Atharva Veda 7.52.1
Finally, whenever we pray through Vedas, we pray for not only us but we pray for everyone. That's why mantras have the plural (nah) in them as opposed to the singular I (aham).
Sham no Mitrah sham Varunah sham Vishnuh sham PrjApatih (1), Sham na
Indro Brihaspatih sham no BhavatvaryamA (2)
May Mitra, Varuna, Vishnu, PrajApati (1), Indra, Brihaspati and Aryama
bestow bliss (sham) on us (2).
Atharva Veda 19.9.6
On us here means on everyone of us.
Similarly, the following mantra clearly asks for all round peace for every possible creatures that exist in the three worlds. Foreigners and everyone else are obviously included in that huge list.
ShantA dhauh shantA prithvi shAntamidamurva antariksham (1), shAntA
udanvatirapah shAntA nah santvoshadhi (2)
May the powers of peace enevelop the earth, heaven, midregion (1),
waters and herbs or the growths of earth (oshadhi) (2).
Atharva Veda 19.9.1
And, the word
Mleccha, loosely speaking, can mean a barbaric person, who consumes forbidden meat, who consumes non sacrificial meat, who does not follow the ethics of SanAtana Dharma and etc. It may not indicate just a foreigner.
And it can also mean someone that you have never thought of.
The Atri Smriti, for example, presents a classification of Brahmins , according to their nature and deeds, into ten groups. And one of them is called the Mleccha.
He, who daily studies the Vedanta, gives up companionship, and
discusses the SAnkhya-Yoga, is called a Dwija. (367)
That Vipra, who, in the very beginning of a battle and before all,
strikes all holders of bow with weapons and defeats [them], is called
a Kshtra. (368)
That Vipra, who is given to agriculture, who tends kine, and who
drives a trade, is called a Vaisya. (369)
That Vipra, who sells shell-lac, salt, saffron, milk, clarified
butter, honey, or meat, is called a Sudra. (370)
A Vipra, who is a thief or a robber, or who is ever prone to give bad
advice or give vent to harsh words, and who is always fond of fish and
flesh, is called a Nishada. (371)
A Vipra who does not know the true nature and being of Brahman but is
always proud of his sacrificial thread, is for that sin called a Pasu
A Vipra, who unhesitatingly obstructs [the use of] tanks, wells,
watery expanses, pleasure-grounds and lakes, is called a Mlechchha.
A Vipra, who is devoid of all religious rites, is ignorant, innocent
of all forms of religions and is cruel to all creatures, is called a