The official language of Laos is Lao, a tonal language closely related to Thai. Thanks to ubiquitous Thai broadcast media most Lao understand Thai fairly well, and some have adopted certain Thai words for tourist use, including farang ("Westerner". Does not apply to foreign Asians).
But it's worth learning a few basic expressions in Lao. The Lao people obviously appreciate that you make an effort even if it is quite limited. French, a legacy of the colonial days, still features on a few signs and is understood by a few people as it used to be a compulsory subject at school. However, the presence of English has also grown in recent years, with many younger people learning it. As a result, youth will generally know some basic English, though proficiency is generally poor.
Tourist areas will sometimes have school children who will practice their English with you as part of their curricular requirements. They may, after a conversation, ask you to sign a form or pose for a photo with you as proof that this conversation took place. These conversations can be a great time to gain some local ideas for your next sightseeing trip.
There are two main ways to turn the Lao script into the Latin alphabet: either French-stylespellings like Houeisay, or English-style spellings like Huay Xai. While government documents seem to prefer the French style, the English spellings are becoming more common. The latter is used on Wikivoyage. Two quick pronunciation tips: Vientiane is actually pronounced "Wieng Chan", and the letter x is always read as an "s".