If you're writing for an English audience, your readers are expecting an English novel. From a reader perspective, it is utterly tedious to read a lot of dialogue you cannot understand. Providing translations can help, but that's equally tedious, since the POV character won't have those translations. I would recommend keeping use of foreign language to a minimum.
If the POV character understands the language, then I would suggest that everything should be written in English with the understanding that they're talking in Conlang, and also that the other characters don't know what is being said.
Example:
"We should take them to the desert and
get rid of them," said Thrull,
switching to Conlang. He glared at the
three tourists. "They're useless."
The tourists blinked and smiled,
confused. "What did he say?" asked the
one with the camera.
"He said he'll take you to the ruins."
replied Tomas. "For a fee."
If the POV character does not understand the language, then I would not necessarily write the dialogue in Conlang, but rather show that the person doesn't understand what's being said. Have the POV rely on the speaker's body language, facial expressions etc. to try pick up what's being said.
Example:
Paul listened as Tomas and the one
called Thrull spoke in their harsh,
guttural tongue. Thrull's lips curled
in a sneer, and he glared at Paul and
his friends as he spat out his final
words.
Paul tried to smile. "What did he
say?"
"He said he'll take you to the ruins,"
replied Tomas. "For a fee."
Paul looked from Tomas to Thrull's
leering eyes. That must be some fee.
When I've overheard people talking in foreign languages, I am completely unable to repeat what was said because of the strangeness of the words, and the speed at which they talk, so it's unrealistic from a POV perspective to have conversations written down in foreign languages.
That's not to say you can't put in some of the language. There are one or two words that get repeated, such as curses, swearwords etc., or short, easily-pronounced phrases, and there you can probably write them as is, because it's something that the character will likely pick up on, even if they don't understand them. Also, sometimes there are written words, signs etc. that the character can repeat. In those cases, the character can ask what they mean, or someone (knowing the character can't understand) can translate for them.