1 Corinthians 14:6-11 KJV - Now, brethren, if I come unto you speaking with tongues, what shall I profit you, except I shall speak to you either by revelation, or by knowledge, or by prophesying, or by doctrine?
7 And even things without life giving sound, whether pipe or harp, except they give a distinction in the sounds, how shall it be known what is piped or harped?
8 For if the trumpet give an uncertain sound, who shall prepare himself to the battle?
9 So likewise ye, except ye utter by the tongue words easy to be understood, how shall it be known what is spoken? for ye shall speak into the air.
10 There are, it may be, so many kinds of voices in the world, and none of them is without signification. 11 Therefore if I know not the meaning of the voice, I shall be unto him that speaketh a barbarian, and he that speaketh shall be a barbarian unto me.
So here is showing that tongues people understand is how God wants it
See verse 5 below for clarification of the verses you quoted.
Paul explains different purposes associated with speaking in tongues. One manifests in personal prayer and needs no interpretation. (14:2) Whereas, when in operation in the church setting, the message in an unknown tongue is to be interpretated by someone operating in the Gift of Interpretation. (14:5) The verses you note reference the problem when mixing the two:
Verse 5:
5 "I would that ye all spake with tongues but rather that ye prophesied: for greater is he that prophesieth than he that speaketh with tongues,
except he interpret, that the church may receive edifying.
The rest of Paul's comment pertains to speaking in tongues without interpretation:
1 Corinthians 14:6-11 KJV - Now, brethren, if I come unto you speaking with tongues, what shall I profit you, except I shall speak to you either by revelation, or by knowledge, or by prophesying, or by doctrine?
7 And even things without life giving sound, whether pipe or harp, except they give a distinction in the sounds, how shall it be known what is piped or harped?
8 For if the trumpet give an uncertain sound, who shall prepare himself to the battle?
9 So likewise ye, except ye utter by the tongue words easy to be understood, how shall it be known what is spoken? for ye shall speak into the air.
10 There are, it may be, so many kinds of voices in the world, and none of them is without signification.
11 Therefore if I know not the meaning of the voice, I shall be unto him that speaketh a barbarian, and he that speaketh shall be a barbarian unto me.